test(frontend): extract CancellableLoader; pin debounce-race invariants in unit tests (0.87.23)

0.87.16 followup. Extract the four debounce-race invariants
(predecessor abort, late-result suppression, AbortError swallow,
ownership-aware finally) from `loadCoverage` into a reusable
`CancellableLoader` helper at $lib/util/cancellable, with 9 unit
tests covering each. Page rewrite uses the helper.

Also corrects: hooks.server.test.ts (added Node-fetch rejection
shape for already-aborted entry; removed redundant tick); admin.test.ts
(AbortError-propagation now uses pre-aborted signal + mockRejectedValueOnce
mirroring real Node fetch).

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

frontend/src/lib/api/admin.test.ts

@@ -731,28 +731,19 @@ describe('admin crawler api client', () => {
     });
 
     it('getAnalysisMangaCoverage propagates an abort rejection', async () => {
-        // Mid-flight abort: caller flips the AbortController, the fetch
-        // promise rejects, and the api client lets the rejection
-        // surface to its caller (the debounce-race handler treats
-        // AbortError as silent cancellation rather than an error toast).
+        // Production Node fetch rejects synchronously with AbortError
+        // when called with an already-aborted signal. Mirror that
+        // (instead of wiring our own listener and asserting the
+        // rejection we self-fire) so the test exercises the contract
+        // the consumer relies on — the CancellableLoader at
+        // $lib/util/cancellable.ts treats AbortError as `null` and
+        // skips the state update.
         const ctrl = new AbortController();
-        fetchSpy.mockImplementationOnce(
-            (_url, init) =>
-                new Promise((_resolve, reject) => {
-                    const signal = (init as RequestInit).signal;
-                    signal?.addEventListener(
-                        'abort',
-                        () => reject(new DOMException('aborted', 'AbortError')),
-                        { once: true }
-                    );
-                })
-        );
-        const inflight = getAnalysisMangaCoverage(
-            { limit: 25 },
-            { signal: ctrl.signal }
-        );
         ctrl.abort();
-        await expect(inflight).rejects.toMatchObject({ name: 'AbortError' });
+        fetchSpy.mockRejectedValueOnce(new DOMException('aborted', 'AbortError'));
+        await expect(
+            getAnalysisMangaCoverage({ limit: 25 }, { signal: ctrl.signal })
+        ).rejects.toMatchObject({ name: 'AbortError' });
     });
 
     it('getAnalysisChapterCoverage unwraps items', async () => {