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/build.gradle.kts

@@ -199,4 +199,34 @@ detekt {
     
     // Parallel-Verarbeitung für schnellere Checks
     parallel = true
+}
+
+// 📋 v1.8.0: Copy F-Droid changelogs to assets for post-update dialog
+// Single source of truth: F-Droid changelogs are reused in the app
+tasks.register<Copy>("copyChangelogsToAssets") {
+    description = "Copies F-Droid changelogs to app assets for post-update dialog"
+    
+    from("$rootDir/../fastlane/metadata/android") {
+        include("*/changelogs/*.txt")
+    }
+    
+    into("$projectDir/src/main/assets/changelogs")
+    
+    // Preserve directory structure: en-US/20.txt, de-DE/20.txt
+    eachFile {
+        val parts = relativePath.segments
+        if (parts.size >= 3) {
+            // parts[0] = locale (en-US, de-DE)
+            // parts[1] = "changelogs"
+            // parts[2] = version file (20.txt)
+            relativePath = RelativePath(true, parts[0], parts[2])
+        }
+    }
+    
+    includeEmptyDirs = false
+}
+
+// Run before preBuild to ensure changelogs are available
+tasks.named("preBuild") {
+    dependsOn("copyChangelogsToAssets")
 }