test(#34): make my-videos mirror relay-truth test self-contained

The mirror flag helpers (nip71MirrorFlags) read the global `localStorage`,
but this test file only wired `globalThis.document` from JSDOM — never
`localStorage`. It therefore passed only when run inside the full suite,
relying on another test file leaking the localStorage shim into the shared
global. Run in isolation it failed 4/4 (isMirrorEnabled always returned
false → "Share to apps" instead of "Shared ✓").

Import the existing setup-localstorage shim so the file is honest in
isolation. No assertions changed — the product code (#34 relay-truth
detection) was already correct; this only removes a load-order dependency.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/my-videos-mirror-relay-truth.test.mjs

@@ -9,6 +9,11 @@
 //   - refreshMirrorStates() looks up relay truth (batched), corrects the button
 //     label, AND reconciles the local flag so it becomes a correct cache.
 
+// Self-contained localStorage shim: the mirror flag helpers read the global
+// `localStorage`, so without this the suite only passed by relying on another
+// test file leaking the shim into the shared global (brittle load-order
+// dependency). Importing it here makes this file honest in isolation.
+import "./test-helpers/setup-localstorage.mjs";
 import test, { beforeEach, afterEach } from "node:test";
 import { strict as assert } from "node:assert";
 import { JSDOM } from "jsdom";