[Widgets] Implement Rust Widget APIs for Charting & Data Visualization

[rorychatt]

Problem Statement

The TypeScript frontend (src/frontend/src/widgets) contains UI renderers for charts, but the Rust rusty crate lacks Rust struct definitions and fluent builders for developer view construction.

Missing Rust Widgets

• AreaChart
• BarChart
• ChordChart
• FunnelChart
• GaugeChart
• LineChart
• PieChart
• RadarChart
• SankeyChart
• ScatterChart

Proposed Architecture & Design

1. Create rusty/src/widgets/charts.rs.
2. Define Rust structs with builder methods (.data(), .x_axis(), .y_axis(), .color_palette(), .legend(), .tooltip()).
3. Re-export all chart widgets in rusty::widgets.

Acceptance Criteria

• All 10 chart widgets have Rust struct definitions and builder APIs.
• Serialized virtual tree JSON matches frontend expectations.
• Unit tests for widget instantiation and serialization.