feat: implement CSS sprite system for image optimization

Reduces HTTP requests from 44+ individual images to 3 sprite sheets
(~93% reduction). Includes Go sprite generator tool, CSS classes,
template integration, and E2E tests.

- Add cmd/sprites/main.go for sprite generation (60x60px + 120x120px @2x)
- Add _sprites.css with responsive sizing and retina support
- Update templates to use sprites with logoIndex fallback
- Add Makefile targets: sprites, sprites-clean
- Add 9-test E2E suite for sprite functionality
- Add doc/22-SPRITES.md with usage documentation

static/css/main.css

@@ -35,6 +35,7 @@
 @import './04-interactive/_toasts.css';
 @import './04-interactive/_zoom-control.css';
 @import './04-interactive/_contact-form.css';
+@import './04-interactive/_sprites.css';
 
 /* 05 - Responsive */
 @import './05-responsive/_breakpoints.css';