feat(chat): add a "Copy my chat address" button

The conversation header only copies a PEER's address, so a user had no way to
hand out their own chat address for others to message them. Add a "Copy my chat
address" link in the left pane (public App::chatMyAddress() forwards the stable
seed-derived reply z-address; copied on click only, with an "address copied"
toast and a warning if none exists yet).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -302,6 +302,9 @@ public:
     wallet::LiteWalletController* liteWallet() { return lite_wallet_.get(); }
     // HushChat service (identity + in-memory message store); the Chat tab reads its store.
     chat::ChatService& chatService() { return chat_service_; }
+    // Public forwarder for the UI: the wallet z-address others use to reach this user in chat. Call on
+    // click (not per-frame) — the underlying chatReplyZaddr() may persist a freshly chosen address.
+    std::string chatMyAddress() { return chatReplyZaddr(); }
     // Why the Chat tab has no identity yet, so it can show the right prompt instead of always
     // "unlock your wallet". Locked = encrypted & locked (seed unreadable); Preparing = unlocked and
     // deriving the identity (fetch in flight, or waiting on the node / address list); Unavailable =