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

templates/partials/sections/experience.html

@@ -14,7 +14,9 @@
         {{range .CV.Experience}}
         <div class="experience-item" id="exp-{{.CompanyID}}" data-company="{{.CompanyID}}" data-title="{{.Company}}" data-position="{{.Position}}">
             <div class="company-logo">
-                {{if .CompanyLogo}}
+                {{if .LogoIndex}}
+                <span class="icon-sprite icon-section icon-company" style="--icon-index: {{.LogoIndex}};" role="img" aria-label="{{.Company}} logo"></span>
+                {{else if .CompanyLogo}}
                 <img src="/static/images/companies/{{.CompanyLogo}}" alt="{{.Company}} logo" onerror="this.parentElement.innerHTML='<iconify-icon icon=\'mdi:office-building\' width=\'80\' height=\'80\' class=\'default-company-icon\'></iconify-icon>'">
                 {{else}}
                 <iconify-icon icon="mdi:office-building" width="80" height="80" class="default-company-icon"></iconify-icon>