Add settings page, PIN manager, and auto-config

- New settings page at /settings (localhost-only) accessible via tray icon
- PIN manager with 6-digit codes, 5-min TTL, single-use
- API token uses secrets.token_urlsafe(32) for 256-bit entropy
- Config auto-creates config.yaml on first run with detected local IP
- Default PIN and token generated randomly on first startup
- Added pin_ttl setting (default 300s)
- Updated build_exe.py for onefile exe with settings assets bundled
- Added start.bat and start.sh for easy development startup

client/index.html

@@ -121,7 +121,7 @@
         <button id="settings-access-btn" data-i18n="access">Access</button>
         <p id="settings-error" class="error"></p>
 
-        <div id="settings-content" style="display: none;">
+        <div id="settings-content" style="display: none">
           <section class="panel">
             <h2 data-i18n="dashboard_pin_label">Dashboard PIN</h2>
             <input