build(webui): CMake target runs npm build into the build tree

cmake/WebUI.cmake

@@ -0,0 +1,28 @@
+# Builds the React web UI via npm at CMake build time, into ${CMAKE_BINARY_DIR}/webui/dist.
+find_program(NPM_EXECUTABLE npm)
+if(NOT NPM_EXECUTABLE)
+    message(WARNING "npm not found — skipping web UI build (set BUILD_WEBUI=OFF to silence)")
+    return()
+endif()
+
+set(WEBUI_SRC  "${CMAKE_SOURCE_DIR}/webui")
+set(WEBUI_DIST "${CMAKE_BINARY_DIR}/webui/dist")
+
+# Install dependencies once (when node_modules is absent).
+add_custom_command(
+    OUTPUT "${WEBUI_SRC}/node_modules/.package-lock.json"
+    COMMAND ${NPM_EXECUTABLE} ci --no-audit --no-fund
+    WORKING_DIRECTORY "${WEBUI_SRC}"
+    COMMENT "webui: npm ci"
+    VERBATIM)
+
+# Build the UI into the CMake build tree (outDir override).
+add_custom_command(
+    OUTPUT "${WEBUI_DIST}/index.html"
+    COMMAND ${NPM_EXECUTABLE} run build -- --outDir "${WEBUI_DIST}" --emptyOutDir
+    WORKING_DIRECTORY "${WEBUI_SRC}"
+    DEPENDS "${WEBUI_SRC}/node_modules/.package-lock.json"
+    COMMENT "webui: npm run build -> ${WEBUI_DIST}"
+    VERBATIM)
+
+add_custom_target(webui ALL DEPENDS "${WEBUI_DIST}/index.html")