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/ui/windows/chat_tab.cpp

@@ -275,6 +275,7 @@ void RenderChatTab(App* app)
         for (const auto& c : convs) if (c.cid == s_selected_cid) { sel = &c; break; }
 
         if (sel) {
+            app->markChatConversationSeen(sel->cid, sel->lastTs);  // viewing the thread clears its unread (Q1)
             // Header: peer name + z-address.
             ImGui::PushFont(material::Type().subtitle1());
             ImGui::TextUnformatted(sel->peerName.c_str());