feat(chat): read-only Chat sidebar tab (Phase 3)

Add the first user-visible HushChat surface: a Chat tab in the sidebar with a
two-pane, read-only conversation view — a conversation list on the left and the
selected thread on the right — reading from the App-owned ChatService store.

- New src/ui/windows/chat_tab.{h,cpp} (RenderChatTab(App*), mirroring the
  Contacts tab). Conversations are sorted by most-recent activity; peer
  z-addresses resolve to contact names via the address book when known; each
  message shows sender + timestamp + wrapped body, with a contact-request tag
  and a read-only footer (composing arrives in a later phase). Empty states
  cover "wallet locked / identity not ready" and "no conversations yet".
- Sidebar wiring: NavPage::Chat + registry entry (static_assert stays balanced),
  NavPageSurface + GetNavIconMD (ICON_MD_CHAT), dispatch in app.cpp, trace/sweep
  page names, and App::chatService() accessor. The tab is gated on the new
  WalletUiSurface::Chat, which returns DRAGONX_ENABLE_CHAT != 0 — so it only
  appears in chat-enabled builds and is hidden (and unreachable) by default.
- i18n: English defaults for the chat nav label + hint strings.

Verified: Linux + Windows(mingw) build with chat ON, ctest 100%, hygiene clean;
caches restored to the OFF default. The screenshot sweep now includes the Chat
tab (sweepPageName "chat") for per-theme visual review.

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

src/app.h

@@ -168,6 +168,8 @@ public:
     config::Settings* settings() { return settings_.get(); }
     // Lite wallet controller (non-null only in lite builds with a linked backend).
     wallet::LiteWalletController* liteWallet() { return lite_wallet_.get(); }
+    // HushChat service (identity + in-memory message store); the Chat tab reads its store.
+    chat::ChatService& chatService() { return chat_service_; }
     // Reason the lite wallet failed to auto-open this session (empty if none / opened OK).
     const std::string& liteOpenError() const { return lite_open_error_; }
     // Show the lite send-time unlock modal (called when a spend is attempted on a locked wallet).