feat(playlists): hybrid table over the drag list, and guard its destructive paths

The column sort and the filters are a VIEW; the playlist's stored order only
changes when the user presses "Save this order", which still goes through the
undoable path. Picking a sort used to write straight to the server, so merely
looking at the list by title rewrote the playlist and marked a YouTube-linked
mirror dirty.

The table:
- playlistColumns.tsx holds the column defs; the body renders them as table rows
  and fills the grip cell itself, since the handle needs the row's sortable
  listeners that a render closure can't reach.
- The # column shows the STORED position, so a view-sort never renumbers the
  playlist under the reader.
- Saving is withheld while a filter is active (it would drop the hidden items),
  and drag is disabled under any view, with the reason spelled out rather than
  silently doing nothing.
- applyView/sameOrder live in lib/playlistView.ts with tests: the sort and the
  channel grouping compose, and getting that wrong produces a plausible order
  that the save button would then persist.
- dnd-kit KeyboardSensor: the grip was focusable and called itself "reorder",
  but only a pointer could move it.

The guards:
- Delete is ONE dialog with every outcome. It was two chained confirms where the
  second one's Cancel/Escape/backdrop meant "delete here only" — dismissing the
  dialog deleted the playlist. ConfirmProvider gained useChoice() for it.
- ConfirmProvider focuses Cancel when an offered action is destructive; the
  confirm button took autoFocus unconditionally, so a stray Enter on an open
  dialog ran "Delete user" or "Clear audit log".
- Removing an item asks first, and rebases the undo history instead of throwing
  it away: useUndoable.rebase maps every snapshot, so the reorder history stays
  valid for the remaining items.
- A failed detail query showed "Loading…" forever; it now says so and offers a
  retry. Rename and reorder failures are surfaced instead of swallowed (reorder
  was a bare .then(), which also produced an unhandled rejection).
- Local reorder/remove no longer require YouTube write scope.
- playlistName() replaces the watch-later localizer copied into three files.

frontend/src/components/AddToPlaylist.tsx

@@ -6,6 +6,7 @@ import { Check, ListPlus, Plus, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
 import { useDismiss } from "../lib/useDismiss";
 import { useScrollFade } from "../lib/useScrollFade";
+import { playlistName } from "../lib/playlistName";
 
 // A small popover (portaled to body, so the feed grid can't clip it) for toggling a
 // video's membership in the user's playlists, with inline "new playlist" creation.
@@ -159,7 +160,7 @@ export default function AddToPlaylist({
                     {pl.has_video && <Check className="w-3 h-3" />}
                   </span>
                   <span className="truncate flex-1 text-left">
-                    {pl.kind === "watch_later" ? t("playlists.watchLater") : pl.name}
+                    {playlistName(pl, t)}
                   </span>
                   {(pl.source === "youtube" || pl.yt_playlist_id) && (
                     <Youtube className="w-3 h-3 shrink-0 text-muted" />