feat: comprehensive WCAG 2.1 AA accessibility audit

- Add aria-labels to menu action buttons (PDF, Print, Contact)
- Add aria-labelledby to toggle checkboxes (desktop + mobile)
- Add -webkit-user-select prefix for Safari compatibility
- Add DynamicCacheControl middleware for HTML pages
- Add accessibility test suite (60-accessibility.test.mjs)
- Add comprehensive accessibility documentation (21-ACCESSIBILITY.md)
- Update Modern Web Techniques doc to mark audit complete

internal/middleware/security.go

@@ -235,3 +235,20 @@ func CacheControl(next http.Handler) http.Handler {
 		next.ServeHTTP(w, r)
 	})
 }
+
+// DynamicCacheControl adds appropriate cache headers for dynamic HTML pages
+// Short cache with must-revalidate for dynamic content
+func DynamicCacheControl(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// For dynamic HTML pages: short cache, must revalidate
+		// This improves performance while ensuring fresh content
+		if os.Getenv("GO_ENV") == "production" {
+			// Production: 5 minutes cache, but must revalidate
+			w.Header().Set("Cache-Control", "public, max-age=300, must-revalidate")
+		} else {
+			// Development: no cache for easier testing
+			w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
+		}
+		next.ServeHTTP(w, r)
+	})
+}