ui: undo toast after trash / archive

A global ToastHost (mounted in the app shell) shows a transient toast with
an Undo action, driven by the ui store (showToast/dismiss/runAction, 5s
auto-dismiss). The notes store fires it centrally: trash → Undo restores;
archive → Undo unarchives — so it works from card, editor, and keyboard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/stores/ui.ts

@@ -1,6 +1,13 @@
 import { defineStore } from "pinia";
 import { ref } from "vue";
 
+interface Toast {
+  id: number;
+  message: string;
+  actionLabel: string;
+  action: () => void;
+}
+
 // Cross-component UI signals that don't belong to any single view (e.g. a
 // global shortcut in the app shell asking the board to act).
 export const useUiStore = defineStore("ui", () => {
@@ -10,5 +17,31 @@ export const useUiStore = defineStore("ui", () => {
     composeTick.value++;
   }
 
-  return { composeTick, requestCompose };
+  // Transient undo toast (e.g. after trash/archive).
+  const toast = ref<Toast | null>(null);
+  let toastTimer: ReturnType<typeof setTimeout> | undefined;
+  let toastSeq = 0;
+
+  function showToast(message: string, actionLabel: string, action: () => void) {
+    toastSeq += 1;
+    const id = toastSeq;
+    toast.value = { id, message, actionLabel, action };
+    clearTimeout(toastTimer);
+    toastTimer = setTimeout(() => {
+      if (toast.value?.id === id) toast.value = null;
+    }, 5000);
+  }
+
+  function dismissToast() {
+    clearTimeout(toastTimer);
+    toast.value = null;
+  }
+
+  function runToastAction() {
+    const action = toast.value?.action;
+    dismissToast();
+    action?.();
+  }
+
+  return { composeTick, requestCompose, toast, showToast, dismissToast, runToastAction };
 });