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.svelte

@@ -2,9 +2,12 @@
   import type { TrackRef } from '$lib/api/types';
   import { formatDuration } from '$lib/media/duration';
   import { player, playQueue, enqueueTrack, playRadio } from '$lib/player/store.svelte';
-  import { Plus } from 'lucide-svelte';
+  import { Plus, Check } from 'lucide-svelte';
   import LikeButton from './LikeButton.svelte';
   import TrackMenu from './TrackMenu.svelte';
+  import {
+    selection, toggleOne, selectRange
+  } from '$lib/selection/store.svelte';
 
   type PlayHandler = (tracks: TrackRef[], index: number) => void;
 
@@ -16,29 +19,45 @@
 
   const track = $derived(tracks[index]);
 
-  // Mirrors QueueTrackRow's "now playing" treatment so the user sees
-  // which row in the album / liked / history / search view is currently
-  // playing without having to read the player bar. Only the accent
-  // border + bg lift — no "Now playing" pill, since the player bar
-  // already names the track.
-  const isCurrent = $derived(player.current?.id === track.id);
+  const isCurrent  = $derived(player.current?.id === track.id);
+  const isSelected = $derived(selection.isSelected(track.id));
+  const selectMode = $derived(selection.active);
 
   function activate() {
     if (onPlay) onPlay(tracks, index);
     else playQueue(tracks, index);
   }
 
-  function onRowClick() {
+  function onRowClick(e: MouseEvent) {
+    // In selection mode any click toggles; outside selection mode click
+    // plays as before. Shift+click extends the range from the anchor
+    // regardless of mode (enters selection mode if nothing was picked).
+    if (e.shiftKey) {
+      e.preventDefault();
+      selectRange(index, tracks);
+      return;
+    }
+    if (selectMode) {
+      toggleOne(track, index);
+      return;
+    }
     activate();
   }
 
   function onRowKey(e: KeyboardEvent) {
     if (e.key === 'Enter' || e.key === ' ') {
       e.preventDefault();
-      activate();
+      if (selectMode) toggleOne(track, index);
+      else activate();
     }
   }
 
+  function onCheckboxClick(e: MouseEvent) {
+    e.stopPropagation();
+    if (e.shiftKey) selectRange(index, tracks);
+    else toggleOne(track, index);
+  }
+
   function onAddClick(e: MouseEvent) {
     e.stopPropagation();
     enqueueTrack(track);
@@ -54,12 +73,34 @@
   role="button"
   tabindex="0"
   aria-label={track.title}
+  data-selected={isSelected}
   onclick={onRowClick}
   onkeydown={onRowKey}
-  class="grid w-full cursor-pointer grid-cols-[32px_1fr_auto_auto_auto_auto_auto] items-center gap-4 px-3 py-2 text-left text-sm odd:bg-surface/50 hover:bg-surface focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent {isCurrent ? 'border-l-2 border-l-accent bg-surface-hover' : ''}"
+  class="group grid w-full cursor-pointer grid-cols-[32px_1fr_auto_auto_auto_auto_auto] items-center gap-4 px-3 py-2 text-left text-sm odd:bg-surface/50 hover:bg-surface focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent {isCurrent ? 'border-l-2 border-l-accent bg-surface-hover' : ''} {isSelected ? 'bg-surface-hover' : ''}"
 >
-  <span class="text-right tabular-nums text-text-secondary">
-    {track.track_number ?? '—'}
+  <span class="relative flex h-6 w-8 items-center justify-end">
+    <!-- Track number: visible by default; hidden when the row is hovered
+         (so the checkbox replaces it) or when this row is selected. -->
+    <span
+      class="tabular-nums text-text-secondary
+             {selectMode || isSelected ? 'hidden' : 'group-hover:hidden'}"
+    >{track.track_number ?? '—'}</span>
+    <!-- Checkbox: hidden by default; revealed on row hover OR whenever
+         the selection bar is active OR this row is the selected one. -->
+    <button
+      type="button"
+      role="checkbox"
+      aria-checked={isSelected}
+      aria-label={isSelected ? `Deselect ${track.title}` : `Select ${track.title}`}
+      onclick={onCheckboxClick}
+      class="flex h-5 w-5 items-center justify-center rounded border
+             {isSelected
+               ? 'border-accent bg-accent text-text-primary'
+               : 'border-border bg-surface text-transparent'}
+             {selectMode || isSelected ? 'flex' : 'hidden group-hover:flex'}"
+    >
+      <Check size={14} strokeWidth={2} />
+    </button>
   </span>
   <span class="truncate">{track.title}</span>
   <LikeButton entityType="track" entityId={track.id} />