Add a GitHub-style activity calendar to the Usage page

[0xroylee]

Summary

• Add a dedicated Usage panel that renders token metrics and a GitHub-style activity calendar
• Aggregate token usage records into daily calendar data and cover the transform with unit tests
• Add the calendar dependencies needed by the web package

Testing

• bun test packages/web/tests/token-usage.test.ts
• bun run check
• bun run typecheck
• bun test
• bun run --filter web build
• Browser-verified the /usage page with mocked token usage data