feat(design): a playlist's one action list (C1) — the row's "…", Delete + undo, the header's "…" (E28 S3e2)

- Playlist rows: the X column became the row's "…" (play, pin, copy title/link, move, order,
  remove). Remove runs at once, with no confirm; its toast's Undo puts the row back at its index.
  The Delete key removes a keyboard-walked row, and focus moves on to the next.
- POST /api/playlists/{id}/items takes an optional `position`, so an undo is one request that
  renumbers the list around the row.
- A new `listDelete` key scope. Only a list with `onDelete` registers it, so the help sheet does not
  offer Delete where nothing handles it.
- A playlist has one list, `usePlaylistActions.items`: rename, pin, YouTube open/push/revert,
  delete. The rail row's "…" and the page header's "…" both open it. Push and revert moved into
  the hook; the header keeps Play all.
- Phone-width playlist table: "#" and Channel are hidden below sm (`Column.phoneHidden`). Their
  fixed widths had left the title no width.

backend/app/routes/playlists.py

@@ -4,7 +4,7 @@ mirror the user's own YouTube playlists in (`/sync-youtube`) and push/revert loc
 out (`/{id}/push-plan`, `/{id}/revert-youtube`), behind the optional write scope."""
 
 from fastapi import APIRouter, Depends, HTTPException
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 from sqlalchemy import func, select, and_
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session, aliased
@@ -507,17 +507,40 @@ def delete_playlist(
     return {"deleted": playlist_id}
 
 
+class AddItemIn(VideoRefIn):
+    # Where the video lands among the list's items, in their stored order (0 = first). Omitted, it
+    # is appended. The list page's "Undo" of a removal puts the row back where it was with this —
+    # ONE request, because append-then-reorder leaves it at the end whenever the second call fails.
+    position: int | None = Field(default=None, ge=0)
+
+
 @router.post("/{playlist_id}/items")
 def add_item(
     playlist_id: int,
-    body: VideoRefIn,
+    body: AddItemIn,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     pl = owned_or_404(db, Playlist, playlist_id, user, what="playlist")
     video_id = body.video_id
     get_or_404(db, Video, video_id, what="video")
-    _add_item(db, pl, video_id, mark_dirty=True)
+    added = _add_item(db, pl, video_id, mark_dirty=False)
+    if added and body.position is not None:
+        # Renumber the whole list with the new row at `position` (clamped to the end): positions
+        # can have gaps after removals, so "index N" is only meaningful over the ordered rows.
+        rows = list(
+            db.execute(
+                select(PlaylistItem)
+                .where(PlaylistItem.playlist_id == pl.id)
+                .order_by(PlaylistItem.position)
+            ).scalars()
+        )
+        new = next(r for r in rows if r.video_id == video_id)
+        rows.remove(new)
+        rows.insert(min(body.position, len(rows)), new)
+        for pos, row in enumerate(rows):
+            row.position = pos
+    recompute_dirty(db, pl)
     db.commit()
     return {"ok": True, "playlist_id": pl.id, "video_id": video_id}