feat: account settings page + dedicated help page

- Add /settings page with password change form and personas list
- Add /help dedicated page (replaces help modal); renders HELP.md with
  collapsible sections, dark theme, back link to active persona
- Add 👤 account button and convert ? button to link in header
- Remove help modal HTML and ~55 lines of modal JS from main app
- Register settings and help routers in main.py

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

cortex/static/index.html

@@ -30,7 +30,8 @@
         <button id="sessions-btn" class="hdr-btn">Sessions</button>
         <button id="files-btn" class="hdr-btn">Files</button>
         <button id="ctx-open-btn" class="hdr-btn" title="Settings">⚙<span class="tier-badge">2</span></button>
-        <button id="help-btn" class="hdr-btn" title="Help &amp; reference">?</button>
+        <a href="/help" class="hdr-btn" title="Help &amp; reference" style="text-decoration:none">?</a>
+        <a id="account-btn" href="/settings" class="hdr-btn" title="Account settings" style="text-decoration:none">👤</a>
         <form method="POST" action="/logout" style="margin:0">
             <button type="submit" class="hdr-btn" title="Sign out" id="logout-btn">⏏</button>
         </form>
@@ -102,17 +103,6 @@
         </div>
     </div>
 
-    <!-- Help modal -->
-    <div id="help-modal">
-        <div id="help-modal-inner">
-            <div id="help-modal-header">
-                <h2>Cortex — Help &amp; Reference</h2>
-                <button class="fm-btn" id="help-close-btn">✕</button>
-            </div>
-            <div id="help-modal-body"></div>
-        </div>
-    </div>
-
     <!-- Auth warning banner — shown when Claude CLI token is near expiry -->
     <div id="auth-banner">
         <div id="auth-banner-text">