fix(lint): clear all 15 oxlint warnings

[aniruddhaadak80]

Summary

Clears all 15 oxlint warnings (npm run lint: 15 warnings / 0 errors before, 0 / 0 after), while preserving the daily reporter's persisted-watermark contract.

• Removes unused catch bindings and verified dead private fields from the browser UI.
• Removes a useless object-spread fallback and fixes optional header access in a test.
• Keeps readWatermark() and wires it into generateDailyReports(): a completed UTC-day report is skipped after restart, while a partial failure writes no watermark and remains retryable.
• Adds focused regression coverage for both watermark paths.

Related issue

Closes AGT-4295.

Type of change

• Bug fix
• New feature
• Refactor / chore
• Docs

Verification

• npm run lint — 0 warnings, 0 errors across 844 files
• npm run typecheck
• npm run build
• Focused tests — 47 passed, including the real browser 403 prompt/retry path and both watermark paths
• Full suite under Node 22 — 5,982 passed, 8 failed, 6 skipped; the same 8 environment/tool-discovery failures reproduce on current main