chore(dashboard): wave 1 of code-health audit fixes

[pratyush618]

Refactors / dedup

• Hoist LEVEL_STYLE/LEVEL_TONE into lib/status.ts as logLevelClass() (was duplicated verbatim in job-logs-tab.tsx and log-stream.tsx).
• Hoist formatAxisTime into features/metrics/utils.ts (was duplicated in throughput-chart.tsx and latency-chart.tsx).
• Extract the DAG BFS layout from job-dag-tab.tsx into a pure dag-layout.ts module so it can be unit-tested without a DOM.

Bugs uncovered while testing

• DAG layer assignment looped forever on cycles. The original BFS lacked a depth bound; cycles in the inputs would push layers up indefinitely. Cap layer depth at nodes.length - 1 and add a regression test.
• Three timestamp call sites multiplied milliseconds by 1000. Backend (now_millis() + the Python timeseries aggregator) emits ms; job-table.tsx, job-replay-tab.tsx, and throughput-chart.tsx were rendering jobs as "in 53,000 years". Other call sites already used the values directly, which is why the bug was inconsistent. Annotate every timestamp field in api-types.ts with /** Unix milliseconds */ so future readers don't re-introduce the same mistake.

Lint hygiene

• Replace the silent eslint-disable-next-line in job-filters.tsx (Biome ignores the comment style) with a proper ref-based debounce that doesn't need a suppression at all.
• Promote correctness/useExhaustiveDependencies from warn to error in biome.json — warns-only rules slip past CI.

URL state

• /dead-letters: page and view are now TanStack Router search params, so deep-linking and back/forward work the same way they do on /jobs.

Test coverage

• lib/api-client.ts — buildUrl (param escaping, null/undefined/"" omission), parse (JSON vs text content-type, ApiError paths), and the public api.{get,post,put,delete} surface.
• lib/errors.ts — isNetworkError against Chrome/Firefox/Safari fetch-failure messages, isServerUnavailable for 502/503/504, and isBackendUnreachable.
• features/settings/derived.ts — parseExternalLinks (parse-guard, type filter, extra-field stripping) and applyJobContext (template substitution, URL-encoding, multi-occurrence).
• features/jobs/components/dag-layout.ts — empty graph, linear chain, fan-out, multi-path depth, cycles, isolated nodes, field preservation.

Cleanup

• Drop unused shortName/docsUrl/repoUrl from lib/site.ts.

Test plan

• pnpm run ci (biome ci + typecheck + 81 vitest tests + vite build) passes locally.
• /dead-letters?page=2&view=flat deep-links straight to that page and view, and back/forward in the browser keeps state.
• Job-table "Created" column and replay-history timestamps render today's dates instead of year-55000 nonsense.
• Throughput chart x-axis matches latency chart x-axis (both in ms now).