feat(v1.8.0): IMPL_06 Post-Update Changelog Dialog

- Add UpdateChangelogSheet.kt with Material 3 ModalBottomSheet
- Show changelog automatically on first launch after update
- Load changelog from F-Droid metadata via assets (single source of truth)
- Add copyChangelogsToAssets Gradle task (runs before preBuild)
- Copy F-Droid changelogs to /assets/changelogs/{locale}/ at build time
- Store last shown version in SharedPreferences (last_shown_changelog_version)
- Add ClickableText for GitHub CHANGELOG.md link (opens in browser)
- Add update_changelog_title and update_changelog_dismiss strings (EN + DE)
- Add KEY_LAST_SHOWN_CHANGELOG_VERSION constant
- Integrate UpdateChangelogSheet in ComposeMainActivity
- Add Test Mode in Debug Settings with "Reset Changelog Dialog" button
- Add SettingsViewModel.resetChangelogVersion() for testing
- Add test mode strings (debug_test_section, debug_reset_changelog, etc.)
- Update F-Droid changelogs (20.txt) with focus on key features
- Add exception logging in loadChangelog() function
- Add /app/src/main/assets/changelogs/ to .gitignore
- Dialog dismissable via button or swipe gesture
- One-time display per versionCode

Adds post-update changelog dialog with automatic F-Droid changelog reuse.
F-Droid changelogs are the single source of truth for both F-Droid metadata
and in-app display. Gradle task copies changelogs to assets at build time.
Users see localized changelog (DE/EN) based on app language.

android/app/src/main/res/values-de/strings.xml

@@ -403,6 +403,10 @@
     <string name="debug_delete_logs">🗑️ Logs löschen</string>
     <string name="debug_delete_logs_title">Logs löschen?</string>
     <string name="debug_delete_logs_message">Alle gespeicherten Sync-Logs werden unwiderruflich gelöscht.</string>
+    <string name="debug_test_section">Test-Modus</string>
+    <string name="debug_reset_changelog">Changelog-Dialog zurücksetzen</string>
+    <string name="debug_reset_changelog_desc">Changelog beim nächsten App-Start anzeigen</string>
+    <string name="debug_changelog_reset">Changelog wird beim nächsten Start angezeigt</string>
     <!-- Legacy -->
     <string name="file_logging_privacy_notice">ℹ️ Datenschutz: Logs werden nur lokal auf deinem Gerät gespeichert und niemals an externe Server gesendet. Die Logs enthalten Sync-Aktivitäten zur Fehlerdiagnose. Du kannst sie jederzeit löschen oder exportieren.</string>
     
@@ -443,6 +447,10 @@
     <string name="about_changelog_title">Changelog</string>
     <string name="about_changelog_subtitle">Vollständige Versionshistorie</string>
     
+    <!-- v1.8.0: Post-Update Changelog Dialog -->
+    <string name="update_changelog_title">Was ist neu</string>
+    <string name="update_changelog_dismiss">Alles klar!</string>
+    
     <!-- v1.8.0: Backup Progress -->
     <string name="backup_progress_creating">Backup wird erstellt…</string>
     <string name="backup_progress_restoring">Backup wird wiederhergestellt…</string>