live: add a control footer under the grid (paging + play/stop + fullscreen)

The live-view <main> now hosts its own 32px footer bar beneath the grid:
prev page on the left, play/stop in the centre, next page + fullscreen on the
right — all icon buttons. Pagination is new: cameras are split into pages of
dim² cells, prev/next walk the pages (page 0 still honours a saved manual
layout), and the arrows disable at the bounds. Fullscreen now targets <main>
so the control bar stays visible in fullscreen (the global stats footer is
outside <main> and hides as before). The sidebar play/fullscreen items now
target <main> too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/index.html

@@ -26,8 +26,20 @@
       <a class="nav-item" href="/settings" data-nav="settings"><span class="ico">⚙</span>Настройки</a>
     </aside>
 
-    <main>
+    <main id="live-main">
       <div class="grid g4" id="grid"></div>
+      <footer class="grid-bar" id="grid-bar">
+        <div class="gb-left">
+          <button class="gb-btn" id="gb-prev" title="Предыдущая страница" aria-label="Предыдущая страница">⏮</button>
+        </div>
+        <div class="gb-center">
+          <button class="gb-btn gb-play" id="gb-play" title="Просмотр / стоп" aria-label="Просмотр / стоп">⏸</button>
+        </div>
+        <div class="gb-right">
+          <button class="gb-btn" id="gb-next" title="Следующая страница" aria-label="Следующая страница">⏭</button>
+          <button class="gb-btn" id="gb-fs" title="Полный экран" aria-label="Полный экран">⛶</button>
+        </div>
+      </footer>
     </main>
   </div>