feat(shopping): toggleCheck (idempotent)

Idempotentes Setzen/Loeschen von shopping_cart_check-Eintraegen
ueber (name_key, unit_key). Check ueberlebt Recipe-Removals,
solange ein anderes Rezept weiterhin zur Zeile beitraegt —
verifiziert durch 3 neue Integrationstests (17 total).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/server/shopping/repository.ts

@@ -107,12 +107,22 @@ export function listShoppingList(
 }
 
 export function toggleCheck(
-  _db: Database.Database,
-  _nameKey: string,
-  _unitKey: string,
-  _checked: boolean
+  db: Database.Database,
+  nameKey: string,
+  unitKey: string,
+  checked: boolean
 ): void {
-  throw new Error('not implemented');
+  if (checked) {
+    db.prepare(
+      `INSERT INTO shopping_cart_check (name_key, unit_key)
+       VALUES (?, ?)
+       ON CONFLICT(name_key, unit_key) DO NOTHING`
+    ).run(nameKey, unitKey);
+  } else {
+    db.prepare(
+      'DELETE FROM shopping_cart_check WHERE name_key = ? AND unit_key = ?'
+    ).run(nameKey, unitKey);
+  }
 }
 
 export function clearCheckedItems(_db: Database.Database): void {