feat: 50 channels, daily 3AM cron sync, countdown timer, remove manual button

frontend/src/routes/YoutubeDashboard.svelte

@@ -8,6 +8,7 @@
   let isSyncing = $state(false);
   let syncMessage = $state('');
   let pendingCount = $state(0);
+  let nextSync = $state(localStorage.getItem('t-yt-next-sync') || '');
   let theme = $state(localStorage.getItem('theme') || 'dark');
   let summaryPanel = $state(null);
   let notes = $state(JSON.parse(localStorage.getItem('t-yt-notes') || '{}'));
@@ -92,10 +93,25 @@
     document.documentElement.className = theme;
     loadVideos();
     loadPendingCount();
+    updateSyncTimer();
     window.addEventListener('keydown', handleKeydown);
     return () => window.removeEventListener('keydown', handleKeydown);
   });
 
+  function updateSyncTimer() {
+    const el = document.getElementById('sync-timer');
+    if (!el) return;
+    const now = new Date();
+    const next = new Date(now);
+    next.setHours(3, 0, 0, 0); // 3 AM daily
+    if (now > next) next.setDate(next.getDate() + 1);
+    const diff = next - now;
+    const h = Math.floor(diff / 3600000);
+    const m = Math.floor((diff % 3600000) / 60000);
+    el.textContent = `Next sync in ${h}h ${m}m`;
+    setTimeout(updateSyncTimer, 60000);
+  }
+
   async function loadPendingCount() {
     try {
       const res = await fetch('/api/videos?status=pending&limit=1');
@@ -191,16 +207,12 @@
       <span class="text-[10px] px-2 py-0.5 rounded font-semibold uppercase tracking-widest" style="background:var(--bg-tertiary);color:var(--text-muted)">text feed</span>
     </div>
     <div class="flex items-center space-x-3">
-      <button onclick={syncSubscriptions} disabled={isSyncing}
-        class="px-3 py-1.5 text-xs font-bold rounded-lg transition"
-        style="background:var(--accent);color:white">
-        {isSyncing ? 'Syncing...' : 'Sync'}
-      </button>
       <button onclick={toggleTheme}
         class="text-sm px-2 py-1 rounded transition" style="color:var(--text-muted)"
         title="Toggle theme">
         {theme === 'dark' ? '☀' : '☾'}
       </button>
+      <span class="text-[10px]" style="color:var(--text-muted)" id="sync-timer"></span>
     </div>
   </header>