fix(web/m7-364): untrack _position in queue immediate-write effect

Wrap the _position read inside untrack() in the immediate-write $effect
so it no longer registers as a reactive dependency, preventing the effect
from firing on every 4Hz position tick. Also adds three missing
restoreQueue unit tests with vi.mock for the auth store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -1,4 +1,5 @@
 import type { TrackRef, RadioResponse } from '$lib/api/types';
+import { untrack } from 'svelte';
 import { api } from '$lib/api/client';
 import { user } from '$lib/auth/store.svelte';
 import { readPersistedQueue, writePersistedQueue } from './persisted';
@@ -379,20 +380,21 @@ $effect.root(() => {
 $effect.root(() => {
   let lastPositionWriteAt = 0;
 
-  // Immediate write on queue or index changes.
+  // Immediate write on queue or index changes. Position is read via
+  // untrack so this effect doesn't fire on every position tick — only
+  // queue/index changes drive it. The throttled-write effect below
+  // covers position drift.
   $effect(() => {
     const userId = user.value?.id;
     if (!userId) return;
-    // Read these to register the dependency. Position is also read but
+    // Reading these registers the reactive dependency; the values
-    // its standalone changes are handled by the second effect below.
+    // themselves are unused — writePersistedQueue re-reads live state.
-    const _q = _queue;
+    _queue;
-    const _i = _index;
+    _index;
-    void _q;
-    void _i;
     writePersistedQueue(userId, {
       queue: _queue,
       index: _index,
-      position: _position
+      position: untrack(() => _position)
     });
     lastPositionWriteAt = Date.now();
   });

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

@@ -14,9 +14,19 @@ import {
   toggleShuffle,
   cycleRepeat,
   reportStateFromAudio,
-  __setShuffleRng
+  __setShuffleRng,
+  restoreQueue
 } from './store.svelte';
 import type { TrackRef } from '$lib/api/types';
+import { writePersistedQueue, clearPersistedQueue } from './persisted';
+
+vi.mock('$lib/auth/store.svelte', () => ({
+  user: {
+    get value() {
+      return { id: 'test-user' };
+    }
+  }
+}));
 
 function track(id: string, dur = 180): TrackRef {
   return {
@@ -546,4 +556,39 @@ describe('queue mutations', () => {
     closeQueueDrawer();
     expect(player.queueDrawerOpen).toBe(false);
   });
+
+  test('restoreQueue rehydrates queue, index, position and parks state at paused', () => {
+    // Seed localStorage manually so we don't depend on the persistence
+    // effect (which is integration-tested at the layout level).
+    const userId = 'test-user';
+    writePersistedQueue(userId, {
+      queue: [track('a'), track('b'), track('c')],
+      index: 1,
+      position: 42.5
+    });
+    restoreQueue(userId);
+    expect(player.queue.map((x) => x.id)).toEqual(['a', 'b', 'c']);
+    expect(player.index).toBe(1);
+    expect(player.position).toBe(42.5);
+    expect(player.state).toBe('paused');
+    clearPersistedQueue(userId);
+  });
+
+  test('restoreQueue clamps out-of-range index', () => {
+    const userId = 'test-user';
+    writePersistedQueue(userId, {
+      queue: [track('a'), track('b'), track('c')],
+      index: 99,
+      position: 0
+    });
+    restoreQueue(userId);
+    expect(player.index).toBe(2);
+    clearPersistedQueue(userId);
+  });
+
+  test('restoreQueue with empty userId is a no-op', () => {
+    playQueue([track('a')], 0);
+    restoreQueue('');
+    expect(player.queue.map((x) => x.id)).toEqual(['a']);
+  });
 });