mobile: one mode-toggle button instead of two bottom tabs

Replace the two-tab bottom bar (Live / Архив) with a single centered
toggle button. It switches the mode either/or: shows the current mode
with a swap glyph (⇆), tap flips Live ⇄ Архив.

VERSION 0.0.150 -> 0.0.151

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

frontend/mobile.html

@@ -36,8 +36,8 @@
     </section>
 
     <nav class="m-tabs">
-      <button class="m-tab active" data-view="live"><span class="i">▦</span>Live</button>
-      <button class="m-tab" data-view="arch"><span class="i">🗄</span>Архив</button>
+      <!-- одна кнопка: переключает режим Live ⇄ Архив (или/или) -->
+      <button class="m-mode" id="m-mode"><span class="i">⇆</span><span class="m-mode-name" id="m-mode-name">Live</span></button>
     </nav>
   </div>
   <script src="/static/mobile.js"></script>