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.

package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "build:app": "tauri build",
     "build:vite": "tsc && vite build",
+    "clean:app": "cd src-tauri && cargo clean",
     "dev:app": "tauri dev",
     "dev:vite": "vite",
     "preview": "vite preview",