feat(chat,notifications): first-run chat intro card; clickable toast history + "+N" badge

- Chat: a dismissible first-run intro card (how chat works — encrypted, sent as
  shielded txs with a small fee, tied to the seed) with a "Copy my chat address"
  button, shown in the empty thread pane; dismissal persisted. The copy logic is
  shared with the existing sidebar button (moved to a helper, unchanged).
- Toasts: clicking a toast now opens the existing notification-history panel, and
  a "+N more" badge appears when several live toasts are queued (render only ever
  showed the newest). Badge opacity is a ui.toml token.

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

src/config/settings.h

@@ -392,6 +392,8 @@ public:
     // Whether the one-time "back up your seed phrase" reminder has already been shown.
     bool getSeedBackupReminded() const { return seed_backup_reminded_; }
     void setSeedBackupReminded(bool v) { seed_backup_reminded_ = v; }
+    bool getChatIntroDismissed() const { return chat_intro_dismissed_; }
+    void setChatIntroDismissed(bool v) { chat_intro_dismissed_ = v; durable_next_save_ = true; }
 
     // Unlock lockout state, persisted across restarts so the failed-attempt curve can't be reset by
     // killing the app. Keyed by the wallet file it applies to; the PIN/passphrase is never stored.
@@ -745,6 +747,7 @@ private:
     std::map<std::string, AddressMeta> address_meta_;
     bool wizard_completed_ = false;
     bool seed_backup_reminded_ = false;
+    bool chat_intro_dismissed_ = false;       // one-time chat "how it works" intro card
     int lock_failed_attempts_ = 0;            // persisted unlock-failure count (lockout curve)
     long long lockout_until_epoch_ = 0;       // unix time the current lockout expires (0 = none)
     std::string lock_state_wallet_;           // wallet file the above two apply to