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)'

backend/library/metrics.go

@@ -54,6 +54,17 @@ type ScanMetrics struct {
 	Warnings []ScanWarning `json:"warnings"`
 }
 
+// ScanProgress is the payload emitted periodically during a scan to
+// report live progress to the frontend.
+type ScanProgress struct {
+	Phase     string `json:"phase"`     // "counting", "scanning", "orphans", "thumbnails"
+	Total     int64  `json:"total"`     // total audio files from pre-walk count
+	Processed int64  `json:"processed"` // added + skipped + updated so far
+	Added     int64  `json:"added"`
+	Skipped   int64  `json:"skipped"`
+	Updated   int64  `json:"updated"`
+}
+
 // ScanWarning represents a non-fatal issue encountered during scanning.
 type ScanWarning struct {
 	FilePath string `json:"filePath"`