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/style.css

@@ -701,3 +701,80 @@ html {
   scroll-behavior: smooth;
 }
 
+
+/* Shown only after something is removed, and only long enough to act on.
+   It carries a real button because Cmd/Ctrl+Z does not exist on a phone,
+   so on touch this is the only way back. */
+.undo-toast {
+  position: fixed;
+  left: 50%;
+  transform: translateX(-50%);
+  bottom: calc(24px + env(safe-area-inset-bottom));
+  z-index: 200;
+  display: flex;
+  align-items: center;
+  gap: 16px;
+  max-width: calc(100vw - 32px);
+  padding: 12px 12px 12px 18px;
+  border-radius: 12px;
+  background-color: var(--bg-surface);
+  border: 1px solid var(--border);
+  box-shadow:
+    0 12px 28px rgba(0, 0, 0, 0.16),
+    0 4px 8px rgba(0, 0, 0, 0.08);
+  font-size: 14px;
+  color: var(--text-primary);
+}
+
+html.dark .undo-toast {
+  box-shadow:
+    0 12px 28px rgba(0, 0, 0, 0.55),
+    0 4px 8px rgba(0, 0, 0, 0.4);
+}
+
+.undo-message {
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.undo-action {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  flex-shrink: 0;
+  padding: 6px 12px;
+  border: none;
+  border-radius: 8px;
+  background: transparent;
+  font-family: inherit;
+  font-size: 14px;
+  font-weight: 500;
+  color: var(--accent-text);
+  cursor: pointer;
+  transition: background-color 200ms ease;
+}
+
+.undo-action:hover {
+  background-color: var(--hover);
+}
+
+/* Teaching the shortcut is the point, so it only shows where one exists. */
+.undo-key {
+  font-family: var(--font-family);
+  font-size: 12px;
+  color: var(--text-secondary);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 1px 5px;
+}
+
+@media (pointer: coarse) {
+  .undo-key {
+    display: none;
+  }
+
+  .undo-action {
+    min-height: 44px;
+  }
+}