calendar: Fix month picker layout (#2764)

## Summary

- render month and year pickers with explicit 3-column and 5-column
grids
- add horizontal gutter spacing without changing the calendar container
width
- use compact typography for month options so longer names fit cleanly
- add regression coverage for picker layout and month-only compact text

## Root cause

The picker used percentage-width items inside a wrapping flex container.
At the default 248px calendar width, item widths plus margins caused the
month view to wrap into two columns. Switching to a grid fixed the
column count, but without a gutter the labels remained visually crowded.

## Validation

- `cargo fmt --check`
- `cargo test -p gpui-base calendar::tests`
- `cargo test -p gpui-component time::calendar::tests`
- `cargo check -p gpui-component`
- `git diff --check HEAD^ HEAD`