feat: Prevent UI hanging during video save or playlist add

Task #1 completed by ClaudeQueue

ClaudeQueue

frontend/async-guard.test.js

@@ -0,0 +1,68 @@
+'use strict';
+
+const { test } = require('node:test');
+const assert = require('node:assert');
+const { runExclusive } = require('./async-guard');
+
+// A controllable promise so a test can hold an operation "in flight".
+function deferred() {
+  let resolve, reject;
+  const promise = new Promise((res, rej) => { resolve = res; reject = rej; });
+  return { promise, resolve, reject };
+}
+
+test('runs fn and returns its resolved value', async () => {
+  const set = new Set();
+  const out = await runExclusive(set, 'a', async () => 42);
+  assert.strictEqual(out, 42);
+  assert.strictEqual(set.size, 0, 'key freed after success');
+});
+
+test('skips a second concurrent call for the same key (fn runs once)', async () => {
+  const set = new Set();
+  let calls = 0;
+  const d = deferred();
+  const first = runExclusive(set, 'k', async () => { calls++; await d.promise; return 'first'; });
+  // Second call while the first is still in flight — must be a no-op.
+  const second = await runExclusive(set, 'k', async () => { calls++; return 'second'; });
+  assert.strictEqual(second, undefined, 'skipped call resolves to undefined');
+  assert.strictEqual(calls, 1, 'fn invoked only once');
+  d.resolve();
+  assert.strictEqual(await first, 'first');
+});
+
+test('different keys run in parallel', async () => {
+  const set = new Set();
+  const dA = deferred();
+  const dB = deferred();
+  let aDone = false;
+  const a = runExclusive(set, 'A', async () => { await dA.promise; aDone = true; return 'A'; });
+  const b = runExclusive(set, 'B', async () => { await dB.promise; return 'B'; });
+  assert.strictEqual(set.size, 2, 'both keys in flight at once');
+  // B can finish before A — proves they are not serialized.
+  dB.resolve();
+  assert.strictEqual(await b, 'B');
+  assert.strictEqual(aDone, false, 'A still in flight while B completed');
+  dA.resolve();
+  assert.strictEqual(await a, 'A');
+  assert.strictEqual(set.size, 0);
+});
+
+test('frees the key after failure so a retry is possible', async () => {
+  const set = new Set();
+  await assert.rejects(
+    runExclusive(set, 'x', async () => { throw new Error('boom'); }),
+    /boom/,
+  );
+  assert.strictEqual(set.has('x'), false, 'key freed even on throw');
+  // Retry now succeeds because the key was released.
+  const out = await runExclusive(set, 'x', async () => 'ok');
+  assert.strictEqual(out, 'ok');
+});
+
+test('supports a synchronous fn', async () => {
+  const set = new Set();
+  const out = await runExclusive(set, 's', () => 7);
+  assert.strictEqual(out, 7);
+  assert.strictEqual(set.size, 0);
+});