fix(chat): wipe typed plaintext on a wallet switch + per-thread draft

Audit B1/B5/B8:
- The chat composer / new-conversation buffers are file-static char[]; on a wallet
  switch or lock the plaintext a user typed for wallet A (a private message, or a
  recipient z-address) resurfaced verbatim in wallet B's composer and lingered
  unwiped in RAM. Add ui::ResetChatTab() (sodium_memzero the buffers + clear the
  selection ids) and call it from App::resetChatSession().
- Wipe the single composer draft when the active conversation changes, so text
  typed for one contact can't be sent to another (B5).
- Refresh the stale "read-only / Phase 3" docs — composing/sending is wired (B8).

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

src/ui/windows/chat_tab.h

@@ -11,17 +11,26 @@ class App;
 namespace ui {
 
 /**
- * @brief Render the Chat tab (read-only HushChat conversation view — Phase 3).
+ * @brief Render the Chat tab (HushChat conversation view).
  *
- * A two-pane view: a conversation list on the left and the selected thread on
- * the right, both read from the App-owned ChatService store (App::chatService()).
- * Peer z-addresses are resolved to contact names via the address book when known.
- * Read-only for now — composing/sending arrives in a later phase. The tab is only
- * reachable when built with DRAGONX_ENABLE_CHAT (gated via WalletUiSurface::Chat).
+ * A two-pane view: a conversation list on the left and the selected thread on the
+ * right, both read from the App-owned ChatService store (App::chatService()). Peer
+ * z-addresses are resolved to contact names via the address book when known.
+ * Composing/sending and starting new conversations are wired (App::sendChatMessage /
+ * startChatConversation). Reachable when built with DRAGONX_ENABLE_CHAT
+ * (gated via WalletUiSurface::Chat).
  *
  * @param app Pointer to the app instance.
  */
 void RenderChatTab(App* app);
 
+/**
+ * @brief Securely wipe the Chat tab's UI-local state (composer / new-conversation
+ *        plaintext buffers + the selected-conversation ids). Called by
+ *        App::resetChatSession() on a wallet switch/lock so one wallet's typed
+ *        plaintext can't resurface (or linger unwiped in RAM) under the next wallet.
+ */
+void ResetChatTab();
+
 } // namespace ui
 } // namespace dragonx