fix: startup crash, watcher count, timeline date fallback

db.rs:
- Move idx_images_taken_at creation to after ensure_column so it never
  runs against a schema where taken_at doesn't exist yet; this was causing
  a startup panic on any DB that predates Phase 1
- COALESCE(taken_at, created_at) → COALESCE(taken_at, modified_at) since
  created_at is never populated (modified_at is always set)
- Remove dead is_tagging_job_cancelled function (superseded by
  is_tagging_job_processing) and unused thumbnail_path from ImagePathRecord

indexer.rs:
- Watcher create path now calls update_folder_count after commit_batch and
  emits folder-counts-changed so the sidebar count stays in sync; the
  notification is only emitted when the DB write actually succeeds

store.ts / Timeline.tsx:
- compareImages taken_asc/taken_desc: fall back to modified_at not
  created_at (created_at is always null)
- Timeline groupImages: same fallback fix so images group by month
  correctly for libraries not yet re-indexed for EXIF
- subscribeToProgress: listen for folder-counts-changed and call
  loadFolders() to keep the sidebar image count live

src/components/Timeline.tsx

@@ -22,7 +22,7 @@ function buildLabel(key: string): string {
 function groupImages(images: ImageRecord[]): TimelineGroup[] {
   const map = new Map<string, ImageRecord[]>();
   for (const img of images) {
-    const ds = img.taken_at ?? img.created_at;
+    const ds = img.taken_at ?? img.modified_at;
     const key = ds ? ds.substring(0, 7) : "unknown";
     let bucket = map.get(key);
     if (bucket === undefined) {