calendar: Fix Calendar styling and FPS monitor layering (#2731)

## Summary

- Align Calendar sizing, typography, spacing, states, and navigation
icons with the shadcn-inspired visual language.
- Keep multi-month headings presentational and non-interactive while
laying them out cleanly.
- Ensure the FPS monitor renders above toast notifications and below the
title bar.

## Validation

- cargo fmt --check
- git diff --check
- cargo check -p gpui-component