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)'
- Add ScanWarning struct with FilePath/Phase/Err to ScanMetrics
- Add mutex-protected addWarning method for concurrent use
- Reclassify walk, extraction, commit, orphan, variant, FTS failures as warnings
- Update commitBatch to return only fatal tx.Commit errors
- Update cachedLinkArtist to check errors via database.IsUniqueViolation
- Update handleConfigUpdate to capture and log scan warning count
yonlu