test(frontend): pin abort-chain + signal pass-through contracts (0.87.16)

Three 0.87.9 follow-ups:

1. hooks.server.ts: two new tests covering the
   already-aborted-at-entry and mid-flight client-disconnect branches
   of the abort chain. Mutation-confirmed.

2. lib/api/admin.ts: two new tests pinning the
   getAnalysisMangaCoverage(init?.signal) pass-through and the
   AbortError-rejection propagation the dashboard's debounce-race
   handler relies on. Mutation-confirmed.

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

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

@@ -713,6 +713,48 @@ describe('admin crawler api client', () => {
         expect(url).toContain('offset=50');
     });
 
+    it('getAnalysisMangaCoverage forwards the AbortSignal to fetch', async () => {
+        // The dashboard's debounce-race fix (0.87.9) relies on this knob
+        // landing on `init.signal`. Without it, a slow first request
+        // can still overwrite a faster second one because `request()`
+        // builds a fresh fetch with no cancellation hook.
+        fetchSpy.mockResolvedValueOnce(
+            ok({ items: [], page: { limit: 50, offset: 0, total: 0 } })
+        );
+        const ctrl = new AbortController();
+        await getAnalysisMangaCoverage(
+            { limit: 50 },
+            { signal: ctrl.signal }
+        );
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.signal).toBe(ctrl.signal);
+    });
+
+    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).
+        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' });
+    });
+
     it('getAnalysisChapterCoverage unwraps items', async () => {
         fetchSpy.mockResolvedValueOnce(
             ok({ items: [{ chapter_id: 'c1', number: 1, title: null, total_pages: 2, analyzed_pages: 1 }] })