feat: add dynamic font resize support

Implement runtime font size and family changes via options proxy,
matching xterm.js API behavior.

Changes:
- Wire up fontSize option changes to renderer.setFontSize()
- Wire up fontFamily option changes to renderer.setFontFamily()
- Add handleFontChange() helper that:
  - Clears active selection (pixel positions changed)
  - Resizes canvas to match new font metrics
  - Forces full re-render
- Add tests for font change behavior

Usage:
  term.options.fontSize = 20;  // Dynamic resize
  term.options.fontFamily = 'Courier New';

Note: Font changes keep cols/rows constant (canvas grows/shrinks).
Use FitAddon.fit() after font change to recalculate grid dimensions.

Author: sreya