feat: opt in to automatically select new apps

[andre487]

Sources now offers Automatically select new apps, off by default. When enabled, newly discovered launcher apps are added to the selected notification sources when Message487 opens or resumes.

Enabling the switch establishes a baseline without selecting existing apps. Persisted discovery history preserves manual exclusions, including after restarts and reinstalling a previously detected app. Disabling automatic selection retains the current selection. Notification capture and Android permission remain separate opt-ins; apps without launcher icons still require manual selection.

Refresh the installed-app list on resume using existing package visibility, without new permissions or background receivers. Add EN/RU labels and explanations, update README and privacy documentation, and cover persistence, self-exclusion, manual exclusions, new-app discovery and the UI toggle.

Validation:

• ./gradlew :app:testDebugUnitTest :app:lintDebug :app:assembleDebug --offline --no-daemon passed: 80 tests, no failures.
• Updated debug APK installed and opened in Message487_API_35 with existing app data preserved.
• git diff --check passed.

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Built from commit d3dc09e9 by CI run #71.
Artifacts expire after 14 days. Neither APK uses the Message487 release key.