fix: keyboard-visible delete, valid list semantics, undo support

The delete button was opacity:0 and only revealed on hover, so tabbing to
it showed nothing at all. The focus outline was being drawn on an
invisible element. It shows on :focus-within now too.

The row was role="button" with tabindex=0 while containing two real
buttons, which isn't valid. Rows are <li> in a <ul> now, so the list gets
announced with its count and position, and the checkbox carries the task
text plus aria-pressed.

Backspace no longer deletes. It's the key people hit meaning "go back",
and there was no way to get the task back afterwards. Delete still works,
and Cmd/Ctrl+Z reverses it.

Enter and Escape move from window to the input. On window, pressing Enter
with a task focused toggled that task and also added whatever was sitting
in the input.

src/style.css

@@ -242,11 +242,18 @@ body {
 }
 
 .task-list {
+  min-height: 400px;
+  position: relative;
+}
+
+/* A real list, so screen readers announce item counts and position. */
+.task-items {
   display: flex;
   flex-direction: column;
   gap: 12px;
-  min-height: 400px;
-  position: relative;
+  list-style: none;
+  margin: 0;
+  padding: 0;
 }
 
 .task-item {
@@ -362,7 +369,11 @@ body {
   flex-shrink: 0;
 }
 
-.task-item:hover .delete-btn {
+/* focus-within matters as much as hover here: the button is opacity 0 by
+   default, so a keyboard user tabbing to it previously saw nothing at all —
+   the focus outline was drawn on an invisible element. */
+.task-item:hover .delete-btn,
+.task-item:focus-within .delete-btn {
   opacity: 1;
 }