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- Remove dead is_tagging_job_cancelled function (superseded by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/store.ts

@@ -498,9 +498,9 @@ function compareImages(a: ImageRecord, b: ImageRecord, sort: SortOrder): number
     case "duration_desc":
       return compareNullableNumber(b.duration_ms, a.duration_ms);
     case "taken_asc":
-      return compareNullableDate(a.taken_at ?? a.created_at, b.taken_at ?? b.created_at);
+      return compareNullableDate(a.taken_at ?? a.modified_at, b.taken_at ?? b.modified_at);
     case "taken_desc":
-      return compareNullableDate(b.taken_at ?? b.created_at, a.taken_at ?? a.created_at);
+      return compareNullableDate(b.taken_at ?? b.modified_at, a.taken_at ?? a.modified_at);
     default:
       return compareNullableDate(b.modified_at, a.modified_at);
   }
@@ -1770,6 +1770,10 @@ export const useGalleryStore = create<GalleryState>((set, get) => ({
       });
     });
 
+    const unlistenFolderCounts = await listen("folder-counts-changed", () => {
+      void get().loadFolders();
+    });
+
     return () => {
       unlistenProgress();
       unlistenMediaJobs();
@@ -1778,6 +1782,7 @@ export const useGalleryStore = create<GalleryState>((set, get) => ({
       unlistenImages();
       unlistenThumbnails();
       unlistenWatcherDeleted();
+      unlistenFolderCounts();
     };
   },
 }));