fix(firewall/ui): Card-Tabs + no-cache Header für index.html

- Tabs type="card": jeder Tab hat eigene sichtbare Box statt Unterstrich
- cardBg: #F1F5F9 für inaktive Card-Tabs
- main.go: index.html + SPA-Fallback mit no-cache Header ausliefern
  (Vite hashed assets /assets/* bleiben immutable=1Jahr cached)
- Verhindert dass der Browser veraltete index.html nach Updates cached

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/edgeguard-api/main.go

@@ -586,10 +586,18 @@ func mountUI(r *gin.Engine) {
 			return
 		}
 		if info, err := os.Stat(full); err == nil && !info.IsDir() {
+			// Vite hashed assets are immutable — cache them forever.
+			// index.html must never be cached so updates take effect.
+			if strings.HasPrefix(clean, "/assets/") {
+				c.Header("Cache-Control", "public, max-age=31536000, immutable")
+			} else {
+				c.Header("Cache-Control", "no-cache, no-store, must-revalidate")
+			}
 			c.File(full)
 			return
 		}
 		// SPA fallback — React Router renders the right page.
+		c.Header("Cache-Control", "no-cache, no-store, must-revalidate")
 		c.File(indexPath)
 	})
 }