desktop: the window installs itself (launcher entry, icon, default.nix)

app/linux/cst-agent-app.{desktop,svg} plus one CMake component ("app") in
app/CMakeLists.txt: `cmake --install --component app` puts the binary in bin/,
the entry in share/applications/ and the icon in hicolor/scalable/apps/. The
entry's file name stays the app_id Qt sets (QGuiApplication::applicationName),
or the launcher draws a generic icon; only the front end installs, the CLI
keeps coming from scripts/bootstrap_linux.sh.

default.nix is that install for nix users: nixpkgs' Qt 6 (qtwayland for a
Wayland session, qtsvg for Breeze's SVG icons), wrapQtAppsHook so the plugins
and QML modules resolve outside a nix-shell and survive nix-collect-garbage,
a filtered src (third_party/ and the build directories are gigabytes this
build never reads), and the version read from include/cst_version.h. doCheck
runs ctest minus test_sandbox, whose Landlock assertion cannot hold inside the
nix build sandbox (the other 40 pass there; make check runs all 41).

Verified: nix-build, nix profile add --file ., then --selftest and --uitest in
a clean environment (no nix-shell, no XDG_* from the session).

app/CMakeLists.txt

@@ -14,6 +14,10 @@ project(cst-agent-app LANGUAGES C CXX VERSION 0.1.0)
 # Inside the repo build (the parent defines the cstcore target, shared with the CLI):
 #   cmake -B build -DCST_BUILD_APP=ON && cmake --build build --target cst-agent-app
 #
+# Desktop install (the window, its launcher entry and the icon; the CLI has
+# its own installer) -- default.nix installs exactly this component:
+#   cmake --install build --component app --prefix ~/.local
+#
 # Extra QML import paths, e.g. a Kirigami installed away from the Qt prefix:
 #   -DCST_APP_QML_IMPORT=...   (added to QML2_IMPORT_PATH / the app's runtime)
 
@@ -119,6 +123,23 @@ if(CST_APP_GUI)
       set_target_properties(cst-agent-app PROPERTIES WIN32_EXECUTABLE TRUE)
     endif()
 
+    # A desktop install. The icon goes where the theme layout says, so the
+    # entry's Icon= name finds it, and the entry's file name is the app_id Qt
+    # sets (QGuiApplication::applicationName) -- rename one and the launcher
+    # loses the icon. All of it in the "app" component: the CLI keeps coming
+    # from scripts/bootstrap_linux.sh, and two `cst-agent` in PATH is how a
+    # version nobody meant to run gets run.
+    if(NOT ANDROID)
+      include(GNUInstallDirs)
+      install(TARGETS cst-agent-app
+              RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT app)
+      install(FILES linux/cst-agent-app.desktop
+              DESTINATION ${CMAKE_INSTALL_DATADIR}/applications COMPONENT app)
+      install(FILES linux/cst-agent-app.svg
+              DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/scalable/apps
+              COMPONENT app)
+    endif()
+
     # --- Android (APK) -----------------------------------------------------
     # The engine is compiled in (top-level CMakeLists: -DCST_BUILD_TUI=OFF,
     # -DCST_BUILD_WEBUI=OFF, so src/webui_stub.c stands in for the server and