Add angular and dotFill props for line style customization

[leanthebean]

Summary

Two new optional props for customizing the line and fill style:

• angular — draws straight line segments between data points instead of the default monotone cubic spline. Useful for financial/portfolio charts where interpolation between data points is undesirable.
• dotFill — replaces the gradient fill under the curve with a stippled dot pattern that fades from top to bottom. Pairs well with angular for a cleaner aesthetic.

Both default to false, preserving existing behavior.

<Liveline
  data={data}
  value={value}
  angular={true}
  dotFill={true}
/>

Changes

• src/types.ts — added angular and dotFill to LivelineProps
• src/draw/line.ts — added drawStraight(), drawDottedFill(), CurveStyle interface; modified renderCurve() to select path/fill strategy
• src/draw/index.ts — threaded curveStyle through DrawOptions to drawLine()
• src/useLivelineEngine.ts — added to EngineConfig, forwarded to draw options
• src/Liveline.tsx — destructured new props, forwarded to engine

All existing tests pass. No breaking changes.