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/app.cpp

@@ -3317,7 +3317,7 @@ void App::renderStatusBar()
                     ImVec2(bellMax.x - r, bellMin.y + r), r, dotCol);
             }
 
-            if (clicked) {
+            if (clicked || notes.consumeHistoryRequest()) {   // bell click, or a click on the toast itself
                 alerts_seen_total_ = notes.totalPushed();  // mark everything currently shown as seen
                 ImGui::OpenPopup("##AlertHistoryPopup");
             }