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>

CMakeLists.txt

@@ -476,6 +476,7 @@ set(APP_SOURCES
     src/ui/windows/qr_popup_dialog.cpp
     src/ui/windows/validate_address_dialog.cpp
     src/ui/windows/contacts_tab.cpp
+    src/ui/windows/chat_tab.cpp
     src/ui/windows/shield_dialog.cpp
     src/ui/windows/request_payment_dialog.cpp
     src/ui/windows/block_info_dialog.cpp
@@ -606,6 +607,7 @@ set(APP_HEADERS
     src/ui/windows/qr_popup_dialog.h
     src/ui/windows/validate_address_dialog.h
     src/ui/windows/contacts_tab.h
+    src/ui/windows/chat_tab.h
     src/ui/windows/contact_picker.h
     src/ui/windows/shield_dialog.h
     src/ui/windows/request_payment_dialog.h