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
-    // its standalone changes are handled by the second effect below.
-    const _q = _queue;
-    const _i = _index;
-    void _q;
-    void _i;
+    // Reading these registers the reactive dependency; the values
+    // themselves are unused — writePersistedQueue re-reads live state.
+    _queue;
+    _index;
     writePersistedQueue(userId, {
       queue: _queue,
       index: _index,
-      position: _position
+      position: untrack(() => _position)
     });
     lastPositionWriteAt = Date.now();
   });