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.cpp

@@ -271,6 +271,7 @@ bool Settings::load(const std::string& path)
         }
         loadScalar(j, "wizard_completed", wizard_completed_);
         loadScalar(j, "seed_backup_reminded", seed_backup_reminded_);
+        loadScalar(j, "chat_intro_dismissed", chat_intro_dismissed_);
         loadScalar(j, "lock_failed_attempts", lock_failed_attempts_);
         loadScalar(j, "lockout_until_epoch", lockout_until_epoch_);
         loadScalar(j, "lock_state_wallet", lock_state_wallet_);
@@ -599,6 +600,7 @@ bool Settings::save(const std::string& path)
     }
     j["wizard_completed"] = wizard_completed_;
     j["seed_backup_reminded"] = seed_backup_reminded_;
+    j["chat_intro_dismissed"] = chat_intro_dismissed_;
     j["lock_failed_attempts"] = lock_failed_attempts_;
     j["lockout_until_epoch"] = lockout_until_epoch_;
     j["lock_state_wallet"] = lock_state_wallet_;