Fix health endpoint HEAD method support, add icon assets

- Allow HEAD requests on /api/v1/health for Docker/CasaOS healthcheck
  compatibility (wget --spider sends HEAD)
- Add project icon in SVG, 192x192 PNG and 512x512 PNG formats

internal/api/handlers/health.go

@@ -43,7 +43,7 @@ func NewHealthHandler(version string, logger interface{ Info(string, ...any) })
 
 // ServeHTTP handles health check requests
 func (h *HealthHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	if r.Method != http.MethodGet {
+	if r.Method != http.MethodGet && r.Method != http.MethodHead {
 		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
 		return
 	}