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/routes/+layout.svelte

@@ -8,7 +8,9 @@
   import { isPublicRoute } from '$lib/auth/publicRoutes';
   import Shell from '$lib/components/Shell.svelte';
   import QueueDrawer from '$lib/components/QueueDrawer.svelte';
+  import SelectionBar from '$lib/components/SelectionBar.svelte';
   import ToastHost from '$lib/components/ToastHost.svelte';
+  import { selection, clearSelection } from '$lib/selection/store.svelte';
   import {
     registerAudioEl,
     reportTimeUpdate,
@@ -104,14 +106,24 @@
 
   $effect(() => {
     const handler = (e: KeyboardEvent) => {
-      if (e.key === 'Escape' && player.queueDrawerOpen) {
-        closeQueueDrawer();
+      if (e.key === 'Escape') {
+        // Selection bar takes priority over queue drawer: pressing Esc
+        // with both open clears the (less-modal) selection first.
+        if (selection.active) clearSelection();
+        else if (player.queueDrawerOpen) closeQueueDrawer();
       }
     };
     document.addEventListener('keydown', handler);
     return () => document.removeEventListener('keydown', handler);
   });
 
+  // Route-change reset: selection state is per-page; navigating away
+  // (or to a different track-list surface) clears whatever was picked.
+  $effect(() => {
+    page.url.pathname; // subscribe
+    clearSelection();
+  });
+
   useMediaSession();
   useEventsDispatcher();
   useGlobalShortcuts();
@@ -156,6 +168,9 @@
   {:else}
     {@render children()}
   {/if}
+  <!-- SelectionBar must be inside the QueryClientProvider so its
+       Like-all action can use useQueryClient. -->
+  <SelectionBar />
 </QueryClientProvider>
 
 <ToastHost />