Fix browser monitoring and deterministic builds

[sarthakagrawal927]

What changed

• remove the dead Web Vitals beacon fallback and its CSP allowance
• initialize PostHog, Web Vitals, and API timing together in the shared provider
• remove the redundant VitalsReporter component
• isolate Playwright Wrangler state and clear generated build/cache outputs before repeat builds
• prevent local browser tests from loading operator .dev.vars

Why

The production smoke found two 404 beacons to a collector that does not exist. Rebuilding locally after the monitoring change also exposed stale OpenNext HTML/chunk hashes retained in Wrangler local state. This fixes both defects without changing production Cloudflare bindings.

Validation

• pnpm lint
• pnpm typecheck
• pnpm docs:check
• pnpm test:coverage (185 passed; 100% selective core coverage)
• pnpm knip:strict
• pnpm build:e2e
• CI=1 CLOUDFLARE_API_TOKEN= pnpm test:e2e (14 passed)
• git diff --check