App 2.6.0: floating button to open the app

The MG4's launcher does not list third-party apps, so the app could
only be opened by reinstalling it. Now a small round MG-red button
floats over the car's screens: tap opens the app, drag moves it (the
position is kept and clamped to the screen), and it hides while the
app itself is open. A switch in the Upload tab turns it off.

The always-on service owns the overlay, so the button is there from
boot. It needs the overlay permission (SYSTEM_ALERT_WINDOW), which the
car build has through the system UID; without it, or if the window
manager refuses, the button stays away and nothing crashes, and the
Upload tab offers the system permission screen.

Tested on an Android 9 AOSP emulator at the head unit's 1920x1080 /
160 dpi with the car APK (installs as UID 1000): button appears over
the home screen, tap opens the app, hidden while open, drag moves it
without opening the app, position survives a force-stop, the switch
works through the UI. FloatingButtonGeometry has 3 unit tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/build.gradle

@@ -34,8 +34,8 @@ android {
         applicationId "com.stinnesbeck.nilsmg4"
         minSdk 27
         targetSdk 36
-        versionCode 62
-        versionName "2.5.0"
+        versionCode 63
+        versionName "2.6.0"
 
         // Nothing about the server ships in the APK: its URL and ingest token are entered in
         // the app's Server tab (ServerConfig), and the database is only reachable by the server.