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/components/TrackRow.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, describe, expect, test, vi } from 'vitest';
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent } from '@testing-library/svelte';
 import type { TrackRef } from '$lib/api/types';
 import { emptyLikesMock } from '../../test-utils/mocks/likes';
@@ -18,12 +18,14 @@ vi.mock('$lib/api/quarantine', () => emptyQuarantineMock());
 
 import TrackRow from './TrackRow.svelte';
 import { playQueue, enqueueTrack, playRadio } from '$lib/player/store.svelte';
+import { selection, clearSelection } from '$lib/selection/store.svelte';
 
 const tracks: TrackRef[] = [
   makeTrack({ title: 'So What', duration_sec: 545 }),
   makeTrack({ id: 't2', title: 'Freddie Freeloader', track_number: 2, duration_sec: 565, stream_url: '/api/tracks/t2/stream' })
 ];
 
+beforeEach(() => clearSelection());
 afterEach(() => vi.clearAllMocks());
 
 describe('TrackRow', () => {
@@ -86,4 +88,40 @@ describe('TrackRow', () => {
       screen.getByRole('button', { name: /track actions for/i })
     ).toBeInTheDocument();
   });
+
+  describe('selection mode', () => {
+    test('clicking the checkbox toggles selection without playing', async () => {
+      render(TrackRow, { props: { tracks, index: 0 } });
+      const box = screen.getByRole('checkbox', { name: /select so what/i });
+      await fireEvent.click(box);
+      expect(selection.isSelected('t1')).toBe(true);
+      expect(playQueue).not.toHaveBeenCalled();
+    });
+
+    test('row click toggles selection (not play) once any row is selected', async () => {
+      render(TrackRow, { props: { tracks, index: 0 } });
+      const box = screen.getByRole('checkbox', { name: /select so what/i });
+      await fireEvent.click(box);
+
+      // Re-render at index 1 — selection mode is now active globally.
+      const { container } = render(TrackRow, { props: { tracks, index: 1 } });
+      const row = container.querySelector('[role="button"][aria-label="Freddie Freeloader"]')!;
+      await fireEvent.click(row);
+      expect(selection.isSelected('t2')).toBe(true);
+      expect(playQueue).not.toHaveBeenCalled();
+    });
+
+    test('shift+click on the row extends the range from the anchor', async () => {
+      render(TrackRow, { props: { tracks, index: 0 } });
+      const box = screen.getByRole('checkbox', { name: /select so what/i });
+      await fireEvent.click(box);
+
+      const { container } = render(TrackRow, { props: { tracks, index: 1 } });
+      const row = container.querySelector('[role="button"][aria-label="Freddie Freeloader"]')!;
+      await fireEvent.click(row, { shiftKey: true });
+      expect(selection.isSelected('t1')).toBe(true);
+      expect(selection.isSelected('t2')).toBe(true);
+      expect(selection.count).toBe(2);
+    });
+  });
 });