fix(nextcloud): NULL parent_id on cross-source-root child folders + spinner

The previous fix NULLed parent_id only for folders within the
SourceRoot being deleted, but folder rows under a *different*
SourceRoot whose path nests inside this one (e.g. 'Leóns 1st Year' at
.../Taco and Muli - 2024 onward/Leóns 1st Year) still pointed into
our delete set. folders_parent_id_fkey kept tripping. Widen the UPDATE
to NULL parent_id for any folder whose parent_id is in folder_ids,
regardless of source_root_id.

UI: trash button on a Nextcloud library now swaps to a spinning
Loader2 while the delete is in flight (only the row being deleted —
others stay as trash icons but disabled). Title updates to flag
that a cascade through every photo + folder can take a few seconds.

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

backend/app/routers/nextcloud.py

@@ -333,14 +333,18 @@ async def delete_nextcloud_source_root(
             )
         )
         # Folders have a self-referential parent_id FK with no
-        # ON DELETE rule. A flat DELETE of the whole subtree trips
-        # `folders_parent_id_fkey` because postgres checks the
-        # constraint per row, regardless of insertion / list order.
-        # NULL the parent_id on every folder we're about to delete
-        # first so the chain breaks cleanly.
+        # ON DELETE rule. NULL parent_id on every folder that points
+        # *into* our delete set — that includes children within this
+        # source root AND any folder under a different SourceRoot whose
+        # path happens to nest inside this one (e.g. a 'Leóns 1st Year'
+        # SourceRoot at `.../Taco and Muli - 2024 onward/Leóns 1st Year`
+        # has folder rows whose parent_id points at folder rows under
+        # 'Taco and Muli - 2024 onward'). Without this, deleting the
+        # outer SourceRoot trips folders_parent_id_fkey from the inner
+        # SourceRoot's still-live rows.
         await db.execute(
             update(Folder)
-            .where(Folder.id.in_(folder_ids))
+            .where(Folder.parent_id.in_(folder_ids))
             .values(parent_id=None)
         )
         await db.execute(delete(Folder).where(Folder.id.in_(folder_ids)))