App 2.18.0: install from a USB stick

Settings -> App -> "Install from USB stick" (car build only). It looks
in the top folder of every mounted volume (found from the app's own
folder on it, since the head unit has no file picker) for
NilsMG4-unstable-<version>.apk, takes the newest one newer than the
installed version, checks it against its .sha256 sidecar and the
package name, and installs it with the same PackageInstaller session
as the over-the-air update. The stable build gets a stub and no button.

Tested on the emulator: 2.18.0 installed a 2.18.1 from the storage's
top folder without a confirmation, as the system user, so the OTA
install step should not need one either.

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 94
-        versionName "2.17.5"
+        versionCode 95
+        versionName "2.18.0"
 
         // 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.