feat: add scan progress bar with phase indicator

Add live progress reporting during library scans:

- Pre-walk count: fast WalkDir to count audio files upfront for
  percentage calculation (~1-2s overhead)
- Progress ticker: emits ScanProgress events every 300ms with
  phase, file counts (added/skipped/updated), and total
- Phase labels: counting → scanning → thumbnails → orphans
- Frontend: progress bar with percentage, file counts breakdown,
  and phase indicator in both config-page and library-manager

Replaces the static 'Scanning...' text with a live progress bar
showing e.g. '62% — Scanning... 1,247 / 2,013 files (891 new,
356 skipped)'

frontend/src/events.ts

@@ -30,6 +30,7 @@ export const Events = {
 
     // Library events
     LibraryScanStarted: "LibraryScanStarted",
+    LibraryScanProgress: "LibraryScanProgress",
     LibraryScanComplete: "LibraryScanComplete",
 } as const;