Use true 60-day availability bars

[VrianCao]

Summary

• Change the public status availability bar to use a true 60-day daily rollup window.
• Render 60 availability slots in the status card and update status-page labels to 60 days.
• Update homepage preload artifacts, local seed data, docs, and snapshot assertions to match the 60-day status bar.

Validation

• pnpm lint
• pnpm typecheck
• pnpm --filter @uptimer/web build
• pnpm --filter @uptimer/worker exec vitest run --config vitest.config.ts --coverage.enabled=false test/public-status.test.ts test/public-homepage-compute.test.ts test/snapshots-public-homepage.test.ts test/internal-sharded-public-snapshot.test.ts test/snapshots-public-monitor-fragments.test.ts test/internal-monitor-fragments-refresh.test.ts

Fixes #86