chore: prepare v1.6.1 release changelogs

- Add F-Droid changelogs (versionCode 15)
  - de-DE: Code-Qualität, Zero Warnings, ktlint, CI/CD
  - en-US: Code quality, Zero warnings, ktlint, CI/CD
  - Both under 500 characters as required

- Update CHANGELOG.md / CHANGELOG.de.md
  - detekt: 29 → 0 issues
  - Build warnings: 21 → 0
  - ktlint reactivated with .editorconfig
  - CI/CD lint checks integrated
  - Constants refactoring (Dimensions, SyncConstants)
  - Preparation for v2.0.0 legacy cleanup

CHANGELOG.md

@@ -8,6 +8,46 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ---
 
+## [1.6.1] - 2026-01-20
+
+### 🧹 Code Quality & Build Improvements
+
+- **detekt: 0 issues** - All 29 code quality issues resolved
+  - Trivial fixes: Unused imports, MaxLineLength
+  - File rename: DragDropState.kt → DragDropListState.kt
+  - MagicNumbers → Constants (Dimensions.kt, SyncConstants.kt)
+  - SwallowedException: Logger.w() added for better error tracking
+  - LongParameterList: ChecklistEditorCallbacks data class created
+  - LongMethod: ServerSettingsScreen split into components
+  - @Suppress annotations for legacy code (WebDavSyncService, SettingsActivity)
+
+- **Zero build warnings** - All 21 deprecation warnings eliminated
+  - File-level @Suppress for deprecated imports
+  - ProgressDialog, LocalBroadcastManager, AbstractSavedStateViewModelFactory
+  - onActivityResult, onRequestPermissionsResult
+  - Gradle Compose config cleaned up (StrongSkipping is now default)
+
+- **ktlint reactivated** - Linting re-enabled with Compose-specific rules
+  - .editorconfig created with Compose formatting rules
+  - Legacy files excluded: WebDavSyncService.kt, build.gradle.kts
+  - ignoreFailures=true for gradual migration
+
+- **CI/CD improvements** - GitHub Actions lint checks integrated
+  - detekt + ktlint + Android Lint run before build in pr-build-check.yml
+  - Ensures code quality on every pull request
+
+### 🔧 Technical Improvements
+
+- **Constants refactoring** - Better code organization
+  - ui/theme/Dimensions.kt: UI-related constants
+  - utils/SyncConstants.kt: Sync operation constants
+
+- **Preparation for v2.0.0** - Legacy code marked for removal
+  - SettingsActivity and MainActivity (replaced by Compose versions)
+  - All deprecated APIs documented with removal plan
+
+---
+
 ## [1.6.0] - 2026-01-19
 
 ### 🎉 Major: Configurable Sync Triggers