fix(version): inject APP_VERSION via Docker build-arg instead of git describe

In Docker builds, .git directory is not available in the container,
so git describe --tags fails and fallback is '0.0.0'. Now:
- build.sh passes --build-arg APP_VERSION=${tag} to frontend build
- Dockerfile accepts ARG APP_VERSION and sets ENV
- vite.config.js checks process.env.APP_VERSION first, then git describe

build.sh

@@ -222,7 +222,7 @@ bundle_release() {
   docker build -f docker/backend.Dockerfile -t "${backend_tag}" .
 
   echo "[bundle] Building frontend image ${frontend_tag}..."
-  docker build -f docker/frontend.Dockerfile -t "${frontend_tag}" .
+  docker build --build-arg "APP_VERSION=${tag}" -f docker/frontend.Dockerfile -t "${frontend_tag}" .
 
   echo "[bundle] Building agent image ${agent_tag}..."
   docker build -f docker/Dockerfile.agent -t "${agent_tag}" .