fix: persist completed item visibility

frontend/src/lib/mvp-utils.ts

@@ -1,3 +1,18 @@
+type BooleanStorage = Pick<Storage, 'getItem' | 'setItem'>
+
+export function readStoredBoolean(storage: BooleanStorage, key: string, fallback: boolean) {
+  try {
+    const value = storage.getItem(key)
+    if (value === 'true') return true
+    if (value === 'false') return false
+  } catch { /* storage may be unavailable */ }
+  return fallback
+}
+
+export function writeStoredBoolean(storage: BooleanStorage, key: string, value: boolean) {
+  try { storage.setItem(key, String(value)) } catch { /* storage may be unavailable */ }
+}
+
 export function dateKey(date: Date) {
   const y = date.getFullYear()
   const m = `${date.getMonth() + 1}`.padStart(2, '0')