feat(web): multi-select + bulk actions on track lists (Tier A3)

Add a global selection store backing a checkbox per track row and a
floating SelectionBar above PlayerBar with Play next / Add to queue /
Add to playlist / Like all / Clear.

- selection/store.svelte.ts: id Set + TrackRef Map + anchor index;
  toggleOne, selectRange (shift+click), clearSelection. Singleton —
  layout effect clears on pathname change.
- TrackRow: checkbox slot replaces the track number on hover; row
  click toggles selection once any row is picked; shift+click extends
  the range. Esc clears (takes priority over closing the queue
  drawer).
- SelectionBar: floating pill above PlayerBar, mounted inside the
  QueryClientProvider so the Like-all action can resolve.
- player.playNextMany: bulk variant of playNext for "Play next" on a
  multi-track selection.

Covered by selection-store unit tests and three new TrackRow tests
(checkbox toggle, sticky select-mode row click, shift+click range).

web/src/lib/player/store.svelte.ts

@@ -288,6 +288,21 @@ export function enqueueTracks(ts: TrackRef[]): void {
   if (_state === 'idle') _index = 0;
 }
 
+// Bulk variant of playNext: inserts every track immediately after the
+// currently-playing one, preserving order. Empty queue seeds the queue
+// with the batch at index 0 (mirrors playNext's single-track shape).
+export function playNextMany(ts: TrackRef[]): void {
+  _radioSeedId = null;
+  if (ts.length === 0) return;
+  if (_queue.length === 0) {
+    _queue = [...ts];
+    _index = 0;
+    return;
+  }
+  const next = _index + 1;
+  _queue = [..._queue.slice(0, next), ...ts, ..._queue.slice(next)];
+}
+
 export async function playRadio(seedTrackId: string): Promise<void> {
   const resp = await api.get<RadioResponse>(
     `/api/radio?seed_track=${encodeURIComponent(seedTrackId)}`