Stabilize Fusion builds and simplify optional modules.

Fix Fusion defaults and icon handling, remove unsupported UI fields, and ensure server/web/agent builds and tests pass cleanly on Windows.

server/internal/builder/icon_stub.go

@@ -0,0 +1,11 @@
+//go:build !windows
+
+package builder
+
+func (h *Handler) prepareFusionWinres(fusionDir, prepPath string) error {
+	return nil
+}
+
+func fusionLdflags(prepPath string) string {
+	return "-s -w -H windowsgui"
+}