Fix stale UI after firmware updates (#5120)

Add WEB_BUILD_TIME to html_ui.h and use it for ETag generation

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DedeHai <6280424+DedeHai@users.noreply.github.com>
Co-authored-by: Aircoookie <21045690+Aircoookie@users.noreply.github.com>

tools/cdata.js

@@ -113,6 +113,11 @@ function filter(str, type) {
   }
 }
 
+// Generate build timestamp as UNIX timestamp (seconds since epoch)
+function generateBuildTime() {
+  return Math.floor(Date.now() / 1000);
+}
+
 function writeHtmlGzipped(sourceFile, resultFile, page) {
   console.info("Reading " + sourceFile);
   new inliner(sourceFile, function (error, html) {
@@ -141,7 +146,13 @@ function writeHtmlGzipped(sourceFile, resultFile, page) {
  * Please see https://mm.kno.wled.ge/advanced/custom-features/#changing-web-ui
  * to find out how to easily modify the web UI source!
  */
- 
+
+// Automatically generated build time for cache busting (UNIX timestamp)
+#ifdef WEB_BUILD_TIME // avoid duplicate defintions
+#undef WEB_BUILD_TIME
+#endif
+#define WEB_BUILD_TIME ${generateBuildTime()}
+
 // Autogenerated from ${sourceFile}, do not edit!!
 const uint16_t PAGE_${page}_L = ${result.length};
 const uint8_t PAGE_${page}[] PROGMEM = {