feat: offer undo at the moment something is removed

Undo existed but nothing said so. The only route was Cmd/Ctrl+Z, which was
documented in a README nobody reads mid-task, and which does not exist on
a phone at all. On touch there was simply no way to get a deleted task
back.

Deleting a task or clearing completed ones now brings up a short-lived bar
with a real Undo button, so touch has a route for the first time. It also
shows the keyboard shortcut, which is the point: someone learns it once,
here, at the only moment they actually care.

It uses the existing tokens, sits out of the way at the bottom, clears
itself after seven seconds and goes as soon as it is used. The shortcut
badge is hidden where no keyboard exists rather than advertising a chord
nobody can press.

src/App.svelte

@@ -62,6 +62,7 @@
     if (index === -1) return;
     undoStack.push({ type: 'delete', task: tasks[index], index });
     setTasks(taskOps.deleteTask(tasks, id));
+    showUndo('Task deleted');
   }
 
   function clearCompleted() {
@@ -74,6 +75,7 @@
     if (removed.length === 0) return;
     undoStack.push({ type: 'clearCompleted', removed });
     setTasks(taskOps.clearCompleted(tasks));
+    showUndo(removed.length === 1 ? '1 completed task cleared' : `${removed.length} completed tasks cleared`);
   }
 
   const viewingToday = $derived(selectedKey === todayKey);
@@ -129,9 +131,35 @@
     tasks = moveTaskToDay(storage, selectedKey, destination, taskId);
   }
 
+  /**
+   * Undo used to be reachable only by Cmd/Ctrl+Z, which nothing announced and
+   * which does not exist on a phone at all. This is the one moment it is worth
+   * saying something: right after work disappears. It carries the shortcut too,
+   * so a keyboard user learns it once, here, rather than from a README.
+   */
+  let undoNotice = $state(null);
+  let undoNoticeTimer = null;
+
+  const UNDO_NOTICE_MS = 7000;
+  const shortcutLabel = /Mac|iPhone|iPad/.test(navigator.platform || navigator.userAgent)
+    ? '\u2318Z'
+    : 'Ctrl+Z';
+
+  function showUndo(message) {
+    undoNotice = message;
+    clearTimeout(undoNoticeTimer);
+    undoNoticeTimer = setTimeout(() => { undoNotice = null; }, UNDO_NOTICE_MS);
+  }
+
+  function dismissUndo() {
+    clearTimeout(undoNoticeTimer);
+    undoNotice = null;
+  }
+
   function undo() {
     const entry = undoStack.pop();
     if (entry) setTasks(applyUndo(tasks, entry));
+    dismissUndo();
   }
 
   function toggleTheme() {
@@ -736,3 +764,12 @@
     </div>
   </main>
 </div>
+
+{#if undoNotice}
+  <div class="undo-toast" role="status" aria-live="polite" transition:fly={{ y: reduceMotion ? 0 : 12, duration: reduceMotion ? 0 : 200, easing: cubicOut }}>
+    <span class="undo-message">{undoNotice}</span>
+    <button class="undo-action" onclick={undo}>
+      Undo <kbd class="undo-key">{shortcutLabel}</kbd>
+    </button>
+  </div>
+{/if}