feat: Claude CLI OAuth token expiry warning

New GET /auth/status endpoint reads ~/.claude/.credentials.json and
returns hours remaining + warning flag. UI shows a dismissible amber
banner when < 24h remain, turning red if expired. Checked on page load
and every 30 minutes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/index.html

@@ -108,6 +108,12 @@
         </div>
     </div>
 
+    <!-- Auth warning banner — shown when Claude CLI token is near expiry -->
+    <div id="auth-banner">
+        <span id="auth-banner-msg"></span>
+        <button id="auth-banner-close" title="Dismiss">✕</button>
+    </div>
+
     <div id="messages"></div>
     <div id="session-id"></div>