feat: add DiceBear avatar proxy with style picker on signup and profile pages

Implement avatar generation via local proxy at `/avatar/{style}/{seed}` that fetches from DiceBear 9.x API with in-memory caching and fallback to initial-based SVG. Add avatar style selection dropdown to signup form and profile edit page, storing `avatar_style` in user records. Update comment rendering to support nested threading with parent-child relationships in post view. Add `avatar_url` and `avatar_styles` template globals, register avatar router in main app, and include avatar CSS classes for rounded image display.

devplacepy/templates/base.html

@@ -28,7 +28,7 @@
                     {% endif %}
                 </a>
                 <a href="/profile/{{ user['username'] }}" class="topnav-user">
-                    <div class="avatar avatar-sm">{{ user['username'][:1].upper() }}</div>
+                    <img src="{{ avatar_url(user.get('avatar_style', 'initials'), user['username'], 32) }}" class="avatar-img avatar-sm" alt="{{ user['username'] }}">
                     <div class="topnav-user-info">
                         <span class="topnav-user-name">{{ user['username'] }}</span>
                         <span class="topnav-user-role">{{ user.get('role', 'Member') }}</span>