feat(chat): unread count + sidebar badge (Q1)

Track a per-conversation "last seen" watermark (message timestamp) on App:
- chatUnreadCount() sums incoming messages newer than each conversation's
  watermark; surfaced as SidebarStatus.chatUnreadCount → a badge on the Chat nav
  item (mirrors the History/Peers badges).
- Viewing a thread marks it seen (markChatConversationSeen while displayed).
- Baseline on load: existing stored messages are marked seen, so only messages
  that arrive while the app is open badge as unread.
- Wiped in resetChatSession so unread state never leaks across a wallet switch.
In-memory only (resets on app restart); persistence is a later refinement.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.cpp

@@ -1503,6 +1503,7 @@ void App::render()
     ui::SidebarStatus sbStatus;
     sbStatus.peerCount   = static_cast<int>(state_.peers.size());
     sbStatus.miningActive = state_.mining.generate || state_.pool_mining.xmrig_running;
+    sbStatus.chatUnreadCount = chatUnreadCount();   // unread badge on the Chat nav item (Q1)
 
     // Load logo texture lazily on first frame (or after theme change)
     // Also reload when dark↔light mode changes so the correct variant shows