feat(ext): shared toast notification system

extension/src/shared/toast.ts

@@ -0,0 +1,26 @@
+export function showToast(
+  message: string,
+  type: 'success' | 'error' | 'info' = 'info',
+  durationMs = 2500,
+): void {
+  let container = document.querySelector<HTMLElement>('.relicario-toast-container');
+  if (!container) {
+    container = document.createElement('div');
+    container.className = 'relicario-toast-container';
+    document.body.appendChild(container);
+  }
+
+  const toast = document.createElement('div');
+  toast.className = `relicario-toast relicario-toast--${type}`;
+  toast.textContent = message;
+  container.appendChild(toast);
+
+  requestAnimationFrame(() => {
+    requestAnimationFrame(() => toast.classList.add('relicario-toast--visible'));
+  });
+
+  setTimeout(() => {
+    toast.classList.remove('relicario-toast--visible');
+    toast.addEventListener('transitionend', () => toast.remove(), { once: true });
+  }, durationMs);
+}