feat: in-app APK update + Gitea CI/CD workflow

- Check /api/app/android/version on launch, prompt user to update
- Download APK and trigger install via FileProvider
- Add Gitea Actions workflow to build release APK and deploy on push
- Add setup script for Act Runner (Java 17 + Android SDK)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/res/xml/file_paths.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths>
+    <external-files-path
+        name="apk_updates"
+        path="." />
+</paths>