v0.3.6: Network indicator, tab badges, message read tracking, UI cleanup

UI:
- Network indicator dot (black/red/yellow/green) + capability labels (Public, Server)
- Tab badges: Feed (new posts), My Posts (new engagement), People (online), Messages (unread)
- Stats bar removed — contextual counts in tab labels
- Message thread popup variable scoping fix

Message read tracking:
- mark_conversation_read on popover open, close, and message send
- Prevents re-notification of already-seen messages after app restart

Network:
- Added has_public_v6(), has_upnp() getters on Network
- NetworkSummaryDto includes hasPublicV6, hasPublicV4, hasUpnp

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/index.html

@@ -18,11 +18,12 @@
   </div>
 
   <header>
-    <h1>ItsGoin</h1>
-    <div id="stats-bar">
-      <span id="stat-posts">0 posts</span> |
-      <span id="stat-peers">0 peers</span> |
-      <span id="stat-follows">0 following</span>
+    <div id="header-row">
+      <h1>ItsGoin</h1>
+      <div id="net-indicator">
+        <span id="net-dot"></span>
+        <span id="net-labels"></span>
+      </div>
     </div>
   </header>