chore(dashboard): wave 2 of code-health audit fixes

[pratyush618]

Summary

Follow-up to PR #102. Picks up two of the three audit items deferred from
that PR:

1. Test coverage for refresh-interval-provider — extracts
   parseRefreshOption() and refreshIntervalMs() as pure helpers and
   adds 6 tests covering the localStorage guard logic (the audit's flagged
   regression risk). Avoids dragging in jsdom + @testing-library/react
   for what is essentially validation of a string union.
2. Re-enables two a11y Biome rules — noStaticElementInteractions and
   useKeyWithClickEvents, both previously toggled off. Running
   biome check confirms zero current violations across all 170 files;
   the rules act as a guard against future div-with-onClick regressions.

Deliberately deferred

The third audit item (hoisting ToggleBtn/ViewToggle out of
routes/dead-letters.tsx and DateInput/msToLocalDatetime out of
features/jobs/components/job-filters.tsx) is not in this PR. Both
have exactly one consumer each — hoisting them now would be premature
abstraction. Worth revisiting only when a second feature actually needs
them.

Test plan

• pnpm run ci (biome ci, typecheck, 87 vitest tests, vite build) clean locally
• CI green