feat: check for updates page in settings

Add a full-screen update checker accessible from a button at the
bottom of the settings page. It queries the fork's GitHub latest
release and can download + install the APK in place.

- UpdateRepository (core/data): fetch releases/latest via GitHub API,
  parse tag/description/APK asset, stream APK download to cacheDir
- VersionComparator (core/domain): compare 4.4.6-ba7opf.N style tags
  (base version first, then build number; v-prefix tolerant) + tests
- IMainContainer/MainContainer: inject updateRepo
- UpdateCheckerScreen (feature/settings): shows current version,
  checks for updates, displays latest version + release notes, and a
  Download & install button; FileProvider hands the APK to the
  system installer
- SettingsScreen: bottom entry button, full-screen page toggle
- AndroidManifest: REQUEST_INSTALL_PACKAGES permission + FileProvider
- i18n strings (en/zh)

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

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths xmlns:android="http://schemas.android.com/apk/res/android">
+    <cache-path name="updates" path="." />
+</paths>