feat: sidebar per-tab chat context, tab bar sync, stop button, UX polish

Extension changes:
- sidepanel.js: per-tab chat history (tabChatHistories map), switchChatTab()
  swaps entire chat view, browserTabActivated handler for instant tab sync,
  stop button wired to /sidebar-agent/stop, pollTabs renders tab bar
- sidepanel.html: updated banner text ("Browser co-pilot"), stop button markup,
  input placeholder "Ask about this page..."
- sidepanel.css: tab bar styles, stop button styles, loading state fixes
- background.js: chrome.tabs.onActivated sends browserTabActivated to sidepanel
  with tab URL for instant tab switch detection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/sidepanel.html

@@ -14,6 +14,9 @@
     </div>
   </div>
 
+  <!-- Browser tab bar -->
+  <div class="browser-tabs" id="browser-tabs" style="display:none"></div>
+
   <!-- Chat Tab (default, full height) -->
   <main id="tab-chat" class="tab-content active">
     <div class="chat-messages" id="chat-messages">
@@ -127,12 +130,13 @@
 
   <!-- Experimental chat banner (shown when chatEnabled) -->
   <div id="experimental-banner" class="experimental-banner" style="display: none;">
-    &#x26A0; Standalone mode &mdash; this is a separate agent from your workspace
+    Browser co-pilot &mdash; controls this browser, reports back to your workspace
   </div>
 
   <!-- Command Bar -->
   <div class="command-bar">
-    <input type="text" class="command-input" id="command-input" placeholder="Message Claude Code..." autocomplete="off" spellcheck="false">
+    <button class="stop-btn" id="stop-agent-btn" title="Stop agent" style="display: none;">&#x25A0;</button>
+    <input type="text" class="command-input" id="command-input" placeholder="Ask about this page..." autocomplete="off" spellcheck="false">
     <button class="send-btn" id="send-btn" title="Send">&#x2191;</button>
   </div>