feat: add Hexagons background component

Adds a new animated hexagonal grid background component, as requested
in #854. Built using the same canvas-based rendering pattern as the
existing Squares component.

Features:
- Flat-top hexagonal grid with seamless tiling animation
- 5 direction options: right, left, up, down, diagonal
- Configurable hex size, border color, and hover fill color
- Smooth animation via requestAnimationFrame
- Mouse hover detection with hex-coordinate mapping
- Radial gradient overlay matching existing backgrounds
- Responsive canvas with resize handling

Includes all 4 component variants (JSX, JSX+Tailwind, TSX, TSX+Tailwind),
demo page with customization controls, and code constants.

Closes #854