test: add Vitest coverage for allowlist

[Rafly0078]

Fixes #71

Adds focused Vitest coverage for the allowlist feature in src/allowlist/index.ts.

/claim #71

[Rafly0078]

Since this is a backend test coverage PR with no UI changes, here is the text output of the test run showing the passing tests for src/allowlist/index.test.ts as my demo evidence:

 ✓ src/allowlist/index.test.ts (6 tests) 15ms
 Test Files  1 passed (1)
      Tests  6 passed (6)