test: snapshot assertions, unit tests, fix login network dependency

- Replace spot-check assertions with deepStrictEqual against expected-snapshot.json
- Add unit tests for API.getStats, API.getRanks (with tiebreak edge cases), Util.post
- Populate admindata.json fixture so /login avoids real GitHub API calls
- Add Cache-Control header tests, persistence verification tests
- Test invalid JSON (400) on all POST endpoints
- Add /add, /getRepositories endpoint tests
- Expose response headers in test helper
- Add cross-endpoint data consistency tests

Author: amitb0ra

tests/e2e/fixtures/admindata.json

@@ -1 +1 @@
-[]
+[{"username":"dhairyashiil","avatarUrl":"https://avatars.githubusercontent.com/u/93669429?v=4"}]

tests/e2e/fixtures/config.json

@@ -7,6 +7,6 @@
   "delay": "10",
   "serverPort": "63111",
   "startDate": "2024-12-01",
-  "contributors": [],
+  "contributors": ["dhairyashiil"],
   "includedRepositories": ["Rocket.Chat"]
 }