ui(sidebar): drop Title field, add bulk notes editor

The Title (user_title) field hadn't earned its place in the sidebar
form — the underlying column stays on the model but the editable
row + its draft state + commit handler are gone.

Bulk Notes: a textarea in the multi-photo bulk panel that replaces
user_notes across the whole selection with one string. Apply commits;
Clear empties the draft without committing. New backend bulk action
'set_notes' validates the value is a string (or null/empty to clear)
and writes to every photo in the selection in one go. Wired through
the standard useBulkPhotoMutations optimistic-patch path, so the
photo cache flips immediately and rolls back on error.

user_notes added to the shared Photo type so patchPhotos accepts the
field; previously it was only on PhotoInfoPanel's local PhotoDetails.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/routers/photos.py

@@ -1252,6 +1252,18 @@ async def bulk_action(
     elif action.action == 'set_color':
         for photo in photos:
             photo.color_label = action.value
+    elif action.action == 'set_notes':
+        # value is the replacement notes string (empty string clears).
+        # Sent verbatim — no whitespace trimming, callers can pre-trim
+        # client-side if they want.
+        if action.value is not None and not isinstance(action.value, str):
+            raise HTTPException(
+                status_code=400,
+                detail="set_notes requires a string value (or null to clear)",
+            )
+        new_notes = action.value or None  # empty string -> null
+        for photo in photos:
+            photo.user_notes = new_notes
     elif action.action in ('set_taken_at', 'set_taken_at_map'):
         # Two shapes share one code path:
         #   set_taken_at      → value is one ISO datetime, applied to every id