feat(duplicates): phased scan progress with skipped-file reporting

Replace the [scanned, total] tuple progress event with a structured
{phase, processed, total, skipped} payload covering all three scan
phases (checking, hashing, confirming). find_duplicates now returns a
DuplicateScanResult with scanned/candidate/skipped counts, and the UI
surfaces phase labels plus a warning when unreadable files were
skipped. Also adds a clean:app script for cargo clean.

CLAUDE.md

@@ -80,7 +80,7 @@ Database: SQLite with WAL mode, stored in the Tauri app data directory as `galle
 | `media-updated` | `ThumbnailBatch` |
 | `caption-model-progress` | `CaptionModelProgress` |
 | `tagger-model-progress` | `TaggerModelProgress` |
-| `duplicate_scan_progress` | `[scanned, total]` |
+| `duplicate_scan_progress` | `{ phase, processed, total, skipped }` |
 
 ### Key types