v0.9.3: notification_send posts on the phone, and Java helpers have a home

The tool was the last Android one that never worked on the NX789J: it would
fail with "Notification.Builder.build() failed: no exception detail" and the
shade stayed empty. The detail was the whole story -- Qt clears a Java
exception before C++ gets to look at it, so a build() that throws and a
build() that returns null look exactly the same from here, and the only way
to catch the difference is to let Java catch it.

- app/android/src/xyz/whcself/cstagent/Notifier.java: the builder, the
  PendingIntent, build() and notify() in one try/catch that returns "" or
  the throwable's class and message. androiddeployqt copies app/android/ with
  the rest of the package source and Qt's Gradle template puts its src/ on
  java.srcDirs, so a class dropped there needs no build-script change (the
  first one is this; the foreground service of the next step goes there too).
- AndroidTools.cpp keeps what it can see from C++ (the context, the channel
  on API 26+, the small icon looked up by name, the launch intent) and calls
  Notifier.post(); the result line now carries the Java failure when there is
  one. The javaErrorText() helper of the previous attempt is gone: reading a
  pending exception from C++ is exactly what does not work here.
- On the device: the tool answers "notification sent", the system holds the
  record (channel cst-agent, importance 3, the framework small icon, the tap
  intent back into the app). Earlier attempts in the same session are in the
  transcript next to it: four failures from the old path, then this one.

Docs: docs/android.md gets the Java-helper mechanism (and why the tool has to
be wrapped in Java at all) and no longer lists notification_send as missing;
app/README.md's platform table and the Android note say where a Java class
goes.

Version 0.9.3.

docs/android.md

@@ -164,17 +164,25 @@ have, so the script rewrites the shebang of the build-tools wrappers.
   being asked for the key again. Rotation keeps the session; the sidebar
   appears in landscape.
 * Platform tools behind `app:android` (`app/src/platform/android/AndroidTools.cpp`):
-  `clipboard_get` / `clipboard_set` under the engine's own tool names and
-  `share` (`ACTION_SEND` + chooser) both work on the device. The file
+  `notification_send`, `clipboard_get` / `clipboard_set` under the engine's
+  own tool names and `share` (`ACTION_SEND` + chooser) all work on the device.
+  The file
   card's **Open** builds a `content://` URI through the app's FileProvider
   (`<package>.qtprovider`, Qt's `@xml/qtprovider_paths`), checks
   `resolveActivity()` before starting `ACTION_VIEW` and grants the reader
   permission; **Share** reuses the share sheet.
-  `notification_send` still fails on the NX789J: the channel and the small
-  icon are found, `Notification.Builder.build()` returns null, and Qt clears
-  the Java exception before C++ can read it ("no exception detail"). Catching
-  it needs the call to be wrapped in Java (a small `app/android/src/...`
-  helper class), which is the next step there.
+* **Java helpers**: a notification built entirely through `QJniObject` posted
+  nothing on the NX789J -- `Notification.Builder.build()` came back null and
+  the tool could only say "failed: no exception detail", because Qt clears a
+  Java exception before C++ gets to look at it. The one step that has to fail
+  visibly (a builder, a `PendingIntent`, `build()`, `notify()`) now lives in
+  `app/android/src/xyz/whcself/cstagent/Notifier.java`, which catches the
+  throwable and hands the text back for the tool result; the device then
+  reported `notification sent` and the shade had it. androiddeployqt copies
+  `app/android/` with the rest of the package source and Qt's Gradle template
+  puts its `src/` on `java.srcDirs`, so **a class dropped in that directory
+  needs no build-script change** -- the front service of the next step goes
+  there too.
 * A session's transcript replays a delivered file (`deliver_file`) as a card
   with Open and Share -- the copy lives next to the session, marked `for_user`
   so no request ever contains it (see `app/README.md`, "Files the agent