chart: Add chart.grid theme color for grid lines

[madcodelife]

Description

Chart grid lines were painted in the theme's border color, the same as the axes, which reads too heavy behind the data. This adds a chart.grid theme color for grid lines only.

• LineChart, AreaChart, BarChart, CandlestickChart and RadarChart grids now use chart_grid. Axes and pie leader lines stay on border, reference lines stay on muted_foreground, so the axis now reads one step above the grid.
• The default theme sets chart.grid to neutral-200/60 (light) and neutral-800/60 (dark).
• A theme that leaves chart.grid unset falls back to border at 60% opacity, so third-party themes get the lighter grid too. It is translucent rather than blended with background, so it sits right on cards and other surfaces.
• .theme-schema.json, the Theme story color list and the Chart docs (en / zh-CN) include the new key.

Public API

gpui-component

• ThemeColor::chart_grid: Hsla — chart grid line color, read from chart.grid in the theme file.
• Theme file key chart.grid — optional; falls back to border at 60% opacity.

How to Test

1. cargo run -- chart and compare the grids in light and dark mode, including grid_columns and the radar chart.
2. cargo test -p gpui-component --lib theme:: covers both an explicit chart.grid and the fallback.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.