v0.9.0: the Qt front end as an Android APK, images in the transcript

Two strands land together: the window draws pictures now, and the whole front
end builds, installs and runs on the phone with the engine linked in-process.

Images (the "make a picture actually show up" part)
- Engine, additively: cst_agent_hooks_t.on_tool_images(call, list, n, ud) fires
  when a call left attachments behind, and cst_agent_pending_rel() /
  cst_agent_pending_is_image() read that list without the app touching the
  store. tests/test_attach.c::test_agent_pending pins the three cases (an
  image, a stored name that passes cst_attach_rel_ok, out-of-range NULL).
- App: image://cstattach/<stored name> (app/src/ui/SessionImages.cpp) serves
  only the current session's attachment directory, checked with
  cst_attach_rel_ok, decodes down to the requested sourceSize and never
  upscales, and holds a mutex because requestImage() runs on the render
  thread while the directory can change underneath it. AgentCore gained
  attachmentDir(), pendingImages(), the toolImages()/promptImages() signals
  and hookToolImages(); history() gives replayed rows their images too.
- QML: ImageStrip.qml (a thumb strip, <=150 px tall, shaped by the picture,
  tooltip with name/size/origin, the file's name when it went missing),
  wired into MessageCard and ToolCard. Qt 6's ListModel cannot hold a
  list-valued role (append() reads back undefined, setProperty drops it), so
  a row carries one image per line as <stored>|<name>|<origin>|<w>x<h> and
  ImageStrip decodes that.

Android (the engine is already in the APK; this is the front end)
- app/src/main.cpp: on Android the workspace, config, tmp and HOME go under
  the app's own data directory before the engine opens anything, because
  QDir::currentPath() is "/" there and /tmp is not writable.
- app/android/: the manifest (QtApplication/QtActivity, INTERNET and
  POST_NOTIFICATIONS, adjustResize, no storage permission -- the picker will
  be the system one), strings, a vector icon; minSdk 24 and targetSdk 35 come
  from app/CMakeLists.txt through QT_ANDROID_MIN/TARGET_SDK_VERSION.
- app/src/platform/android/AndroidTools.cpp: the Android file behind
  PlatformInit(), owner "app:android" (notification, clipboard, share sheet,
  file picker are stubs for now, so the tool list does not lie about them).
- app/tools/selftest.c decides its scratch workspace with os_temp_dir(), so
  the C smoke test runs on the device too.
- scripts/build_apk_android.sh: sources to installed APK in one command
  (environment checks, qt-cmake configure, --target apk, adb install), and
  the host-side plumbing the NixOS + qt.io combination needs -- the NDK
  sysroot include dirs for Qt's FindGLESv2/FindEGL (they only look under
  <sysroot>/include), -L$PREFIX/lib on the shared/module link lines as well
  as the executables', the Android debug keystore created on first use with
  QT_ANDROID_SIGN_APK + QT_ANDROID_KEYSTORE_* (androiddeployqt copies the
  unsigned gradle output and signs it itself), and the #!/bin/bash shebang of
  the build-tools wrappers, which NixOS does not have.
- scripts/gradle-android-init.gradle (installed into ~/.gradle/init.d/): the
  release lint task is switched off -- lintVitalAnalyzeRelease resolves its
  own tool jars straight from dl.google.com, bypassing every repository the
  build configures, and fails the build after all the real work is done.
- Icons: app/icons/*.svg (Breeze's 24@3x files, the 19 names the window asks
  for) are compiled in as :/cst-icons and ThemeIcons.cpp falls back to them
  when the platform has no icon theme. On the phone the Qt theme lookup found
  nothing and the QML glyph fallback was an empty box in the phone's fonts.
- docs/android.md: "The APK (Qt front end)" -- the SDK assembled without
  sdkmanager, the host Qt's missing libraries on NixOS, the Gradle mirrors,
  lint, signing -- and a Status that says what runs where. app/README.md
  gains the images and APK lines, and stops claiming Android platform tools
  that are still stubs.

Verified: engine `make check` green; desktop `cst-app-selftest: OK` and
--uitest all checks pass; phone (NX789J, Android 16, arm64-v8a) -- the APK
installs, starts on the workspace under /data/user/0/xyz.whcself.cstagent/
files/, a turn reaches a configured provider over HTTPS from inside the ART
process and renders the reply, rotation keeps the session and docks the
sidebar, and the toolbar/card icons draw.

Version 0.9.0.

mode = coding

docs/android.md

@@ -38,6 +38,95 @@ nix-shell -p cmake ninja curl android-tools --run '
 `--run-tests` builds the whole `tests/` suite, pushes it and runs every binary
 on the phone, printing a pass/fail line per test.
 
+## The APK (Qt front end)
+
+`scripts/build_apk_android.sh [--install] [--logs] [--clean]` configures with
+the Qt for Android kit, builds the `apk` target (Gradle, no port opened) and
+optionally installs it. Bits it needs, all overridable through the
+environment:
+
+| variable | what it points at |
+| --- | --- |
+| `QT_ANDROID` | Qt for Android 6.11.2 (`aqt install-qt all_os android 6.11.2 android_arm64_v8a -O ~/Qt --archives qtbase qtdeclarative qtquickcontrols2`) |
+| `QT_HOST` | the host Qt of the same version (`aqt install-qt linux desktop 6.11.2 linux_gcc_64 -O ~/Qt --archives qtbase qtdeclarative qtsvg`) |
+| `ANDROID_SDK_ROOT` | SDK with `platforms/android-<n>` and `build-tools/<v>` |
+| `ANDROID_NDK_HOME` | the NDK the engine was built with |
+| `JAVA_HOME` | JDK 17+ for Gradle |
+
+### The SDK without sdkmanager
+
+`nixpkgs`'s full `androidsdk` pulls the emulator and system images (gigabytes);
+the two components actually needed are cheap. The platform comes from the nix
+store (unfree, so the license flag is required) and build-tools from Google's
+own zip, laid out as the SDK `sdkmanager` would:
+
+```sh
+PLATFORM=$(NIXPKGS_ALLOW_UNFREE=1 NIXPKGS_ACCEPT_ANDROID_SDK_LICENSE=1 nix build \
+  --impure --no-link --print-out-paths --expr \
+  'builtins.elemAt (import <nixpkgs> {}).androidenv.androidPkgs.platforms 2')   # android-35
+mkdir -p ~/android-sdk/{build-tools,platforms}
+ln -s $PLATFORM/libexec/android-sdk/platforms/android-35 ~/android-sdk/platforms/
+```
+
+`androiddeployqt` then picks `build-tools` and `platform-tools` from there. For
+build-tools, `dl.google.com` is the only source: from here it hands out
+~100 KB/s per connection, so the script downloads the zip in 16 ranged
+requests (≈25x faster) and fixes the exec bits Python's `zipfile` drops.
+
+### Host Qt on NixOS
+
+The host Qt is built for a regular distribution: its tools (`qmlimportscanner`
+first of all) look for `libgssapi_krb5.so.2`, `libglib-2.0.so.0`, `libz.so.1`,
+`libfontconfig`, `libX11`, `libEGL`/`libGL`, `libzstd`, `libbrotlidec`,
+`libdbus-1`, `libfreetype`, `libstdc++`, none of which exist in NixOS's search
+paths. The tools carry `RUNPATH=$ORIGIN/../lib`, so symlinking those libraries
+from the nix store into `<QT_HOST>/lib/` is enough:
+
+```sh
+nix build --no-link --print-out-paths nixpkgs#krb5.lib nixpkgs#dbus.lib \
+  nixpkgs#libglvnd nixpkgs#fontconfig.lib nixpkgs#freetype nixpkgs#glib.lib \
+  nixpkgs#libx11 nixpkgs#libxkbcommon nixpkgs#zlib nixpkgs#zstd \
+  nixpkgs#brotli.lib nixpkgs#stdenv.cc.cc.lib      # then ln -s <lib> ~/Qt/6.11.2/gcc_64/lib/
+```
+
+Two more things the script does on the way:
+
+* **GLESv2/EGL**: Qt's `FindGLESv2.cmake` only searches `CMAKE_FIND_ROOT_PATH`
+  prefixes (`<sysroot>/include`), but the NDK keeps the headers in
+  `<sysroot>/usr/include` -- without `-DGLESv2_INCLUDE_DIR`/`-DEGL_INCLUDE_DIR`
+  `find_package(Qt6 Gui)` fails with "dependency GLESv2 could not be found".
+* **`-L$PREFIX/lib` on every link type**: the APK's payload is a *shared
+  module*, so `CMAKE_SHARED_LINKER_FLAGS`/`CMAKE_MODULE_LINKER_FLAGS` need the
+  prefix too, not just the EXE flags.
+
+### Gradle
+
+The generated project resolves plugins and dependencies from `google()` and
+`mavenCentral()`, plus a Gradle distribution from `services.gradle.org`. From
+here all three are slow (dl.google.com ~100 KB/s), so two local tweaks:
+
+* `~/.gradle/init.d/*.gradle` (machine-local, not in the repo) prepends the
+  Aliyun/Tencent mirrors for `pluginManagement`, `dependencyResolutionManagement`
+  and every `buildscript`.
+* the wrapper in `<QT_ANDROID>/src/3rdparty/gradle/gradle/wrapper/` points at
+  the Tencent Gradle mirror (51 MB/s vs 100 KB/s).
+
+`scripts/gradle-android-init.gradle` (installed into `~/.gradle/init.d/` by the
+build script) switches off release lint: `lintVitalAnalyzeRelease` resolves its
+own tool jars straight from `dl.google.com`, bypassing every repository the
+build configures, and fails the build after all the real work is done.
+
+### Signing
+
+`androiddeployqt` copies the **unsigned** Gradle output and signs it itself
+(`QT_ANDROID_SIGN_APK` + the `QT_ANDROID_KEYSTORE_*` variables; a signed Gradle
+output makes it fail with "Could not copy unsigned APK"). The script exports
+those variables and creates the standard Android debug keystore
+(`~/.android/debug.keystore`, alias `androiddebugkey`) on first use;
+`CST_ANDROID_KEYSTORE` and `CST_ANDROID_KEY_ALIAS`/`_PASS` override it.
+`apksigner` is a bash script with a `#!/bin/bash` shebang, which NixOS does not
+have, so the script rewrites the shebang of the build-tools wrappers.
+
 ## What is Android-specific
 
 * **API level 24** (`API_LEVEL=24`): bionic provides threads, `exec` and
@@ -66,8 +155,13 @@ on the phone, printing a pass/fail line per test.
 * The whole suite runs on the device (`scripts/build_android.sh arm64-v8a
   --run-tests`): 36/36 after the diff test learned to tell toybox's `diff` from
   GNU diffutils.
-* Still to come: the Qt front end packaged as an APK. nixpkgs has no Qt for
-  Android build, so that needs a Qt host kit plus the `android_arm64_v8a` kit
-  (qt.io or aqtinstall), a JDK and the Android SDK platform/build-tools;
-  `app/src/platform/android/` (JNI tools, owner `app:android`) and the Gradle
-  project under `app/android/` do not exist yet.
+* The APK builds, installs and runs: the Qt front end links the engine
+  in-process (no port), `main.cpp` puts the workspace, config and tmp under
+  `/data/user/0/xyz.whcself.cstagent/files/`, and a turn does a real HTTPS
+  request from inside the ART process (with no API key the provider answers
+  401 -- the engine, curl and TLS parts of the round trip are what that
+  shows). Rotation keeps the session; the sidebar appears in landscape.
+* Still to come: the Android platform tools behind `app:android`
+  (`notification_send`, clipboard, share sheet, SAF file picker), the back
+  gesture, and the icon glyphs -- `ThemeIcon` finds no icon files on the phone
+  and draws empty boxes where the desktop shows Breeze icons.