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/ui/sidebar.h

@@ -26,6 +26,7 @@ enum class NavPage {
     Receive,
     History,
     Contacts,
+    Chat,
     // --- separator ---
     Mining,
     Market,
@@ -53,6 +54,7 @@ inline const NavItem kNavItems[] = {
     { "Receive",   NavPage::Receive,  nullptr,    "receive",  nullptr     },
     { "History",   NavPage::History,  nullptr,    "history",  nullptr     },
     { "Contacts",  NavPage::Contacts, nullptr,    "contacts", nullptr     },
+    { "Chat",      NavPage::Chat,     nullptr,    "chat",     nullptr     },
     { "Mining",    NavPage::Mining,   "TOOLS",   "mining",   "tools"     },
     { "Market",    NavPage::Market,   nullptr,    "market",   nullptr     },
     { "Console",   NavPage::Console,     "ADVANCED","console",  "advanced"  },
@@ -81,6 +83,7 @@ inline wallet::WalletUiSurface NavPageSurface(NavPage page)
     case NavPage::Receive:  return wallet::WalletUiSurface::Receive;
     case NavPage::History:  return wallet::WalletUiSurface::History;
     case NavPage::Contacts: return wallet::WalletUiSurface::Contacts;
+    case NavPage::Chat:     return wallet::WalletUiSurface::Chat;
     case NavPage::Mining:   return wallet::WalletUiSurface::Mining;
     case NavPage::Market:   return wallet::WalletUiSurface::Market;
     case NavPage::Console:  return wallet::WalletUiSurface::Console;
@@ -107,6 +110,7 @@ inline const char* GetNavIconMD(NavPage page)
     case NavPage::Receive:   return ICON_MD_CALL_RECEIVED;
     case NavPage::History:   return ICON_MD_HISTORY;
     case NavPage::Contacts:  return ICON_MD_CONTACTS;
+    case NavPage::Chat:      return ICON_MD_CHAT;
     case NavPage::Mining:    return ICON_MD_CONSTRUCTION;
     case NavPage::Market:    return ICON_MD_TRENDING_UP;
     case NavPage::Console:   return ICON_MD_TERMINAL;