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.

fastlane/metadata/android/en-US/changelogs/20.txt

@@ -1,12 +1,13 @@
-🎉 v1.8.0 — WIDGETS, SORTING & SYNC IMPROVEMENTS
+🎉 v1.8.0 — WIDGETS & UI POLISH
 
 • New: Home screen widgets with interactive checkboxes
+• New: Widget opacity & lock settings
 • New: Note sorting (date, title, type)
-• New: Checklist auto-sort (unchecked first/checked last)
-• New: Server deletion detection for multi-device sync
-• New: Sync status legend (help button explains all icons)
-• Improved: Live sync progress with phase indicators
-• Improved: Parallel downloads for faster sync
-• Improved: Checklist overflow gradient for long text
-• Fixed: Drag & drop flicker in checklists
-• Fixed: Widget text notes scrolling
+• New: Parallel downloads (1-10 simultaneous)
+• Improved: Grid view as default layout
+• Improved: Sync settings reorganized into clear sections
+• Improved: Live sync progress with status indicators
+• More UI/UX improvements
+
+Full changelog:
+https://github.com/inventory69/simple-notes-sync/blob/main/CHANGELOG.md