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/app_network.cpp

@@ -34,6 +34,7 @@
 #include "rpc/rpc_worker.h"
 #include "rpc/connection.h"
 #include "chat/chat_identity.h"  // deriveChatIdentityFromSecret for HushChat identity provisioning
+#include "ui/windows/chat_tab.h"  // ui::ResetChatTab — wipe chat UI plaintext on a wallet switch
 #include <sodium.h>              // sodium_memzero for wiping the fetched mnemonic
 #include <cctype>
 #include "config/settings.h"
@@ -2672,6 +2673,7 @@ void App::resetChatSession()
     // Drop identity + decrypted plaintext in RAM, lock the seed-encrypted DB, re-arm provisioning.
     chat_service_.clearIdentity();
     chat_service_.store().clear();
+    ui::ResetChatTab();  // wipe the chat UI's typed plaintext + selection so it can't leak into the next wallet
     chat_db_.lock();
     chat_identity_provisioned_ = false;
     chat_identity_fetch_in_flight_ = false;

src/ui/windows/chat_tab.cpp

@@ -2,8 +2,8 @@
 // Copyright 2024-2026 The Hush Developers
 // Released under the GPLv3
 //
-// chat_tab.cpp — read-only HushChat view (Phase 3): a conversation list + the
-// selected thread, both read from the App-owned ChatService store.
+// chat_tab.cpp — HushChat view: a conversation list + the selected thread (both read
+// from the App-owned ChatService store), with a composer and a new-conversation flow.
 
 #include "chat_tab.h"
 #include "../../app.h"
@@ -15,6 +15,8 @@
 #include "../layout.h"   // Layout::dpiScale()
 #include "imgui.h"
 
+#include <sodium.h>      // sodium_memzero — wipe typed plaintext on a wallet switch
+
 #include <algorithm>
 #include <cfloat>
 #include <ctime>
@@ -32,6 +34,7 @@ std::string s_scroll_to_cid;  // when set, scroll the thread to the bottom next
 
 // Composer + new-conversation UI state.
 char s_compose[512] = "";
+std::string s_compose_cid;   // the conversation s_compose is a draft for; draft is wiped when it changes
 bool s_show_new_convo = false;
 char s_new_zaddr[128] = "";
 char s_new_msg[256] = "";
@@ -132,6 +135,12 @@ void RenderChatTab(App* app)
         s_selected_cid = convs.front().cid;
         s_scroll_to_cid = s_selected_cid;
     }
+    // The composer holds a single draft — wipe it when the active conversation changes so text typed
+    // for one contact can't be sent to another (B5).
+    if (s_selected_cid != s_compose_cid) {
+        sodium_memzero(s_compose, sizeof(s_compose));
+        s_compose_cid = s_selected_cid;
+    }
 
     const ImVec2 avail = ImGui::GetContentRegionAvail();
     const float listW = std::clamp(avail.x * 0.32f, 220.0f, 360.0f);
@@ -318,5 +327,18 @@ void RenderChatTab(App* app)
     }
 }
 
+void ResetChatTab()
+{
+    // Securely wipe typed plaintext (a private message / recipient z-addr) so it can't resurface under
+    // the next wallet after a switch/lock, and drop the selection ids.
+    sodium_memzero(s_compose, sizeof(s_compose));
+    sodium_memzero(s_new_zaddr, sizeof(s_new_zaddr));
+    sodium_memzero(s_new_msg, sizeof(s_new_msg));
+    s_selected_cid.clear();
+    s_scroll_to_cid.clear();
+    s_compose_cid.clear();
+    s_show_new_convo = false;
+}
+
 } // namespace ui
 } // namespace dragonx

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