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/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.87.15",
+  "version": "0.87.16",
   "private": true,
   "type": "module",
   "scripts": {