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/util/i18n.cpp

@@ -211,6 +211,12 @@ void I18n::loadBuiltinEnglish()
         strings_["transactions"] = "Transactions";
         strings_["history"] = "History";
         strings_["contacts"] = "Contacts";
+        strings_["chat"] = "Chat";
+        strings_["chat_locked_hint"] = "Unlock your wallet to load your chats.";
+        strings_["chat_empty_hint"] = "No conversations yet. Messages you receive will appear here.";
+        strings_["chat_you"] = "You";
+        strings_["chat_contact_request"] = "contact request";
+        strings_["chat_readonly_note"] = "Read-only — sending messages arrives in a later update.";
         strings_["contacts_search_placeholder"] = "Search contacts...";
         strings_["contacts_search_no_match"] = "No matching contacts";
         strings_["address_book_confirm_delete"] = "Confirm delete?";