feat: add database indexes, update CSS container styles, and enhance RPC query security

- Add unique index on username column in user table and composite indexes on channel_member and channel_message tables during app initialization
- Introduce .container CSS class with flex layout, padding, and nested styling for links and lists in base.css
- Change PWA manifest scope from /web.html to / to cover entire app
- Remove commented-out YouTube embed code from template.py and add .jpg to supported image extensions
- Rename "Chat Rooms" heading to "Channels" in app.html template
- Restructure search_user.html template with proper indentation and container class for form and results
- Strip sensitive fields (email, password, message, html) from RPC query results before returning them

src/snek/templates/app.html

@@ -34,7 +34,7 @@
   <main>
     {% block sidebar %}
     <aside class="sidebar">
-      <h2 class="no-select">Chat Rooms</h2>
+      <h2 class="no-select">Channels</h2>
       <ul>
         {% for channel in channels %}
         <li><a class="no-select" href="/channel/{{channel['uid']}}.html">{{channel['name']}}</a></li>