feat: detailed step-by-step status in sidebar loading screen

Replace useless "Connecting..." with real-time debug info:
- "Looking for browse server... (attempt N)"
- Shows port, server responding status, token status
- Shows chrome.runtime errors if extension messaging fails
- Tells user to run /open-gstack-browser if server not found

extension/sidepanel.html

@@ -22,7 +22,8 @@
     <div class="chat-messages" id="chat-messages">
       <div class="chat-loading" id="chat-loading">
         <div class="chat-loading-spinner"></div>
-        <p>Connecting...</p>
+        <p id="loading-status">Looking for browse server...</p>
+        <pre id="loading-debug" class="muted" style="font-size:11px; font-family:'JetBrains Mono',monospace; white-space:pre-wrap; margin-top:8px; color:#71717A;"></pre>
       </div>
       <div class="chat-welcome" id="chat-welcome" style="display:none">
         <div class="chat-welcome-icon">G</div>