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- Remove dead is_tagging_job_cancelled function (superseded by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-tauri/src/indexer.rs

@@ -1480,8 +1480,17 @@ fn process_watcher_path(
 
         match commit_batch(pool, &[record]) {
             Ok(committed) if !committed.is_empty() => {
+                // Always emit the images — they are committed to the DB.
                 emit_images(app, &IndexedImagesBatch { folder_id, images: committed });
                 emit_folder_job_progress(app, pool, &[folder_id], false);
+                // Update the sidebar count only if we successfully write the new
+                // count; skip the frontend notification on pool/DB failure to
+                // avoid showing a stale number.
+                if let Ok(count_conn) = pool.get() {
+                    if db::update_folder_count(&count_conn, folder_id).is_ok() {
+                        let _ = app.emit("folder-counts-changed", ());
+                    }
+                }
             }
             Ok(_) => {}
             Err(e) => eprintln!("Watcher: commit error for {:?}: {}", path, e),
@@ -1494,6 +1503,7 @@ fn process_watcher_path(
                 if db::delete_images_by_ids(&conn, &[image_id]).is_ok() {
                     db::update_folder_count(&conn, folder_id).ok();
                     let _ = app.emit("watcher-deleted", vec![image_id]);
+                    let _ = app.emit("folder-counts-changed", ());
                 }
             }
             Ok(None) => {} // never indexed or already removed