feat(playlists): make YouTube-mirrored playlists editable + syncable

Mirrors (source='youtube') were read-only; per the original two-way design they can
now be edited locally (add/remove/reorder/rename) and pushed back. The read-sync
skips a dirty mirror so it won't clobber unpushed edits; a successful push clears
dirty and lets the mirror refresh. _mark_dirty now covers any YouTube-linked list
(exported local or mirror), and rename marks dirty too. push/delete no longer reject
mirrors. Push reconciles the title as well (cheap playlists.list compare, then
playlists.update only if changed) so a local rename doesn't revert on the next pull.
Frontend: editable = not-built-in (was also excluding mirrors); rows/reorder enabled
for all owned lists; AddToPlaylist popover lists mirrors too (with a YT marker); the
origin chip now reads 'edits sync back'. Trilingual.

backend/app/youtube/client.py

@@ -247,6 +247,27 @@ class YouTubeClient:
             raise YouTubeError(f"{method} {path} -> {resp.status_code}: {resp.text[:300]}")
         return resp.json() if resp.content else {}
 
+    def get_playlist_snippet(self, playlist_id: str) -> dict | None:
+        """The snippet (title/description) of one playlist, or None if it no longer
+        exists. 1 unit (OAuth, so private playlists work)."""
+        data = self._get(
+            "playlists",
+            {"part": "snippet", "id": playlist_id, "maxResults": 1},
+            cost=1,
+            allow_key=False,
+        )
+        items = data.get("items", [])
+        return items[0].get("snippet") if items else None
+
+    def update_playlist_title(self, playlist_id: str, title: str) -> None:
+        """Rename a playlist on YouTube. 50 units."""
+        self._write(
+            "PUT",
+            "playlists",
+            params={"part": "snippet"},
+            json={"id": playlist_id, "snippet": {"title": title[:150]}},
+        )
+
     def create_playlist(self, title: str, description: str = "", privacy: str = "private") -> str:
         """Create a new YouTube playlist; returns its id. 50 units."""
         data = self._write(