feat: New Lit Chart components and updated gallery demo surfaces

[chrismarino]

Description

Add an interactive Lit Chart component (pie, doughnut, bar) with one-level drill-down support using Chart.js and chartjs-plugin-datalabels. Register the component with a full JSON schema in the component registry.

Also includes items omitted from earlier commits:

• Add OrgChart demo surface and data model to gallery_examples.py (component existed but had no gallery entry)
• Enable custom elements on gallery surfaces (.enableCustomElements)
• Add chart and org-chart demo entries to DEMO_ITEMS list
• Add chart.js and chartjs-plugin-datalabels dependencies

Update documentation:

• Rewrite component gallery README with architecture overview, setup steps, and custom component authoring guide
• Rewrite custom-components README with full property docs for all four components (Chart, OrgChart, PremiumTextField, WebFrame)
• Update path-resolution and registration examples for current API

Test

1. Build the renderer (new dependency):

cd renderers/lit
npm install
npm run build

2. Start the agent (in one terminal):

cd samples/agent/adk/component_gallery
uv run .

This runs on http://localhost:10005.

3. Start the client (in another terminal):

cd samples/client/lit/component_gallery
npm install
npm run dev

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces new interactive Chart components (pie, bar, doughnut) for the Lit renderer, along with corresponding demo surfaces in the component gallery. It also includes updates to the OrgChart demo and significantly improves the documentation for custom components.

The implementation of the chart component is comprehensive, with support for drill-down functionality. My review focuses on improving the maintainability, theming, and type safety of the new code. Specifically, I've suggested refactoring the verbose demo data generation, using theme variables for chart colors, and improving type safety in the Chart.js integration.

A key concern is the lack of automated tests for the new ChartComponent. Given its complexity and the repository's style guide, adding unit tests is highly recommended to ensure correctness and prevent future regressions.