feat: add customizable chart colors via props

[dtunikov]

Description

This PR adds the ability to customize chart colors (markColor, markColorFade, etc.) by passing a chartColors prop to the EmbeddingAtlas component. This provides flexibility for theming and branding while maintaining backward compatibility with default colors.
This PR also adds theme property to EmbeddingAtlas react component, that let the clients customise embeddings view and table themes from outside.

Motivation

Users may want to customize the chart colors to match their application's theme or branding. Currently, the chart colors are hardcoded in the chartColors object, making it impossible to override them without modifying the source code.

Changes

• Added ChartColors interface and exported it from api.ts for TypeScript support
• Added chartColors prop to EmbeddingAtlasProps interface
• Created getChartColors() helper function to merge custom colors with defaults
• Updated ChartContext to include customChartColors field
• Updated all chart components (CountPlot, CountPlotList, Histogram, HistogramStack, BoxPlot) to use getChartColors()
• Connected the prop through EmbeddingAtlas.svelte to the chart context

Example Usage

import { EmbeddingAtlas } from 'embedding-atlas';

const atlas = new EmbeddingAtlas(container, {
  coordinator,
  data: {
    table: "my_table",
    id: "id",
    projection: { x: "x", y: "y" },
  },
  chartColors: {
    light: {
      markColor: "#ff6b6b",      // Custom red instead of default blue
      markColorFade: "#ffe0e0",  // Light red
    },
    dark: {
      markColor: "#ff6b6b",
      markColorFade: "#4d2020",
    }
  }
});

Testing

• ✅ Build passes successfully (npm run build)
• ✅ All files compile without TypeScript errors
• ✅ Backward compatible - existing code works without any changes
• ✅ Custom colors properly override defaults when provided

Checklist

• Code follows the project's style guidelines
• Changes are backward compatible
• Build passes successfully
• TypeScript types are properly defined and exported