feat: Implement actual WebGPU chart rendering

Replace placeholder chart rendering logic with full WebGPU implementation:

- Add embedded WGSL shaders for candlestick and line charts
- Implement GPU pipeline creation for both chart types
- Add data buffer management for OHLCV and point data
- Implement uniform buffer creation with proper alignment
- Add automatic data range calculation for axes
- Implement bind group creation for shader resources
- Add proper background clearing with configurable colors
- Support candlestick series with customizable colors
- Support line series with optional fill
- Add comprehensive rendering logic traversing scene graph

The chart renderer now properly initializes GPU resources on first
render, creates storage buffers for chart data, uploads uniforms for
chart properties, and executes instanced draw calls for efficient
rendering of large datasets.

All pre-commit checks pass including format, vet, tests, and builds.

Author: claude