Add stage 5: list detail page with items UI and drag reorder

API: new PUT /api/lists/{id}/items/order takes the full ordered id set and
rewrites positions 0..n-1 in a transaction (422 unless the set matches the
list exactly). TodoItemRepository gains idsForList() and reorder().

Frontend: lists on the home page are now links to /lists/:id (ListView).
ListView shows the list title, a "M of N done" summary, and each item as a
drag handle + checkbox + inline-editable text (saved on blur) + delete
button, with a create-item form at the bottom. Drag-and-drop uses
vuedraggable; on drop the whole order is persisted via the new endpoint and
the response replaces local state, with a resync-on-error fallback. New
items store; items store is also reset on logout.

Tests: reorder happy path, incomplete-set rejection, owner scoping. Backend
suite: 23 passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Repository/TodoItemRepository.php

@@ -106,6 +106,49 @@ final class TodoItemRepository
         $this->pdo->prepare('DELETE FROM todo_items WHERE id = :id')->execute(['id' => $id]);
     }
 
+    /**
+     * IDs of every item in the list, in current position order.
+     *
+     * @return int[]
+     */
+    public function idsForList(int $listId): array
+    {
+        $stmt = $this->pdo->prepare(
+            'SELECT id FROM todo_items WHERE list_id = :list ORDER BY position ASC, id ASC'
+        );
+        $stmt->execute(['list' => $listId]);
+
+        return array_map(intval(...), $stmt->fetchAll(PDO::FETCH_COLUMN));
+    }
+
+    /**
+     * Assign positions 0..n-1 to the given items in one transaction.
+     *
+     * @param int[] $orderedIds every item id in the list, exactly once
+     * @return TodoItemRow[] the list's items in their new order
+     */
+    public function reorder(int $listId, array $orderedIds): array
+    {
+        $stmt = $this->pdo->prepare(
+            'UPDATE todo_items SET position = :position,
+                    updated_at = ' . "strftime('%Y-%m-%dT%H:%M:%SZ', 'now')" . '
+             WHERE id = :id AND list_id = :list'
+        );
+
+        $this->pdo->beginTransaction();
+        try {
+            foreach (array_values($orderedIds) as $position => $id) {
+                $stmt->execute(['position' => $position, 'id' => $id, 'list' => $listId]);
+            }
+            $this->pdo->commit();
+        } catch (\Throwable $e) {
+            $this->pdo->rollBack();
+            throw $e;
+        }
+
+        return $this->allForList($listId);
+    }
+
     private function nextPosition(int $listId): int
     {
         $stmt = $this->pdo->prepare(