Add Image Download Support (New Download Type)

## Summary This PR adds a new IMAGE download type to Seal, allowing users to download video thumbnails and playlist covers at various quality levels. ## Features Added ### 1. New Download Type: IMAGE - Added IMAGE to DownloadType enum - Integrated into download dialog workflow - Accessible via download type selector ### 2. ImageSelectionPage UI - Material 3 design with checkboxes for thumbnail/playlist cover selection - Quality selector with radio buttons: - Low (default.jpg) - Medium (mqdefault.jpg) - High (hqdefault.jpg) - HD (sddefault.jpg) - Original (maxresdefault.jpg - default) - Confirmation dialog before download - Toast notifications for user feedback ### 3. Smart Quality Selection - Implemented `pickBestImageUrl()` with HTTP HEAD request probing - Automatic quality fallback: ORIGINAL → HD → HIGH → MEDIUM → LOW - Pre-validates URLs before enqueuing downloads - Only downloads images that actually exist (no 404 errors) ### 4. Robust Download Implementation - Retry logic: 2 attempts with 1-second delay - Timeout configuration: 5s for HEAD probes, 30s for downloads - Proper Content-Type validation (`image/*`) - File extension extracted from Content-Type header - Clean filename generation: `<videoId>_<quality>.<ext>` ### 5. File Management - Saves to `/Pictures/Seal/` directory - MediaScanner integration for gallery apps - Proper stream management and resource cleanup ## Technical Details ### Modified Files - `PreferenceUtil.kt` - Added IMAGE enum value - `DownloadDialogViewModel.kt` - Added ImageQuality enum and state management - `ImageSelectionPage.kt` - New UI component (327 lines) - `DownloaderV2.kt` - Image download orchestration - `DownloadUtil.kt` - pickBestImageUrl() and downloadImageFromUrl() - `FileUtil.kt` - getPicturesSealFile() helper - `Task.kt` - ImageDownload TypeInfo - `DownloadDialogV2.kt` - Image type routing and error handling - `strings.xml` - Image-related strings ### Bug Fixes - Fixed ErrorPage crash when handling FetchImages errors - Fixed duplicated filename suffixes (e.g., "maxresmaxresdefault.jpg") - Fixed file extension mismatches ## Testing Tested with real YouTube thumbnails: - ✅ Old videos with limited quality availability (automatic fallback works) - ✅ Modern videos with all quality levels (uses requested quality) - ✅ HEAD request validation (200 status + image/* Content-Type) - ✅ Gallery integration (MediaScanner refresh) - ✅ Clean filenames with correct extensions ## Checklist - [x] Code follows project style guidelines - [x] Added/updated relevant documentation - [x] Tested on Android 15 (API 35) - [x] No breaking changes - [x] All existing tests pass - [x] Added appropriate error handling