Add Test a link, Portuguese localization, and update APK cleanup

[neteinstein]

Description

Three related Settings/UX improvements:

• Test a link (feature:settings, feature:share, app): a new field in Settings, shown
  right after the Gemini API key is saved, lets you paste an Instagram/TikTok link and run it
  through the exact same pipeline as a real OS share - no need to switch back to Instagram/TikTok
  first. Since feature modules never depend on each other directly, this is wired through
  MainActivity (the composition root) into ShareRoute's existing sharedText mechanism. That
  surfaced a latent bug: LaunchedEffect doesn't restart when the same URL is submitted twice in a
  row (Compose skips recomposition on structurally-equal state), fixed with a sharedTextRequestId
  nonce bumped alongside sharedText on every submission.
• Portuguese localization (app + all feature modules, core locale config): full
  values-pt/strings.xml translations across every module with user-facing strings, verified
  key-for-key/array-count/format-specifier parity against the English source. Added
  android:localeConfig + locales_config.xml (en, pt) so Android 13+ automatically exposes an
  "App language" entry in system Settings - no extra runtime code needed given targetSdk=36. A
  new Language section sits at the top of Settings (ahead of the API key field, per explicit
  request) with a button deep-linking to Settings.ACTION_APP_LOCALE_SETTINGS, hidden below API 33.
• Update APK cleanup (core:update): after feature:settings's "Update to latest" installs a
  newer build, the downloaded APK was left behind in context.cacheDir/updates/. Added
  ClearDownloadedUpdateUseCase + a manifest-registered UpdateApkCleanupReceiver listening for
  ACTION_MY_PACKAGE_REPLACED (a documented exception to Android 8+ implicit-broadcast
  restrictions, delivered only to the just-updated app and survives process death during install)
  to delete it once install finishes.

agents.md and README.md are updated to document all three.

How to Test

• ./gradlew ktlintCheck
• ./gradlew test (covers the 3 new ClearDownloadedUpdateUseCaseTest cases)
• ./gradlew compileDebugKotlin compileDebugUnitTestKotlin
• Manual: save a Gemini API key in Settings, confirm the "Test a link" field appears, paste an
  Instagram/TikTok link, tap Test, and confirm it runs through the share pipeline. Confirm the new
  Language section is at the top of Settings and deep-links correctly on API 33+. Install an update
  via "Update to latest" and confirm the downloaded APK is gone from the cache afterward.

Checklist

• ./gradlew ktlintCheck passes (or ktlintFormat was run to fix style)
• ./gradlew test passes for all affected modules
• New/changed logic has unit test coverage (domain and data layers especially)
• agents.md / README.md updated if this changes architecture, setup, or dev workflow
• No secrets, API keys, or credentials committed