Fix: motivate browsers to reload web app resources

by mangling the Git commit hash into the MD5 hash of all web application
resource's ETag headers, we ensure that index.html is essentially
invalidated in the browser's cache, even if the actual content is the
same between firmware versions. this hopefully makes the browser
recognize that it has to reload app.js in particular.

src/WebApi_webapp.cpp

@@ -4,6 +4,7 @@
  */
 #include "WebApi_webapp.h"
 #include <MD5Builder.h>
+#include <__compiled_constants.h>
 
 extern const uint8_t file_index_html_start[] asm("_binary_webapp_dist_index_html_gz_start");
 extern const uint8_t file_favicon_ico_start[] asm("_binary_webapp_dist_favicon_ico_start");
@@ -24,6 +25,12 @@ void WebApiWebappClass::responseBinaryDataWithETagCache(AsyncWebServerRequest* r
     auto md5 = MD5Builder();
     md5.begin();
     md5.add(const_cast<uint8_t*>(content), len);
+
+    // ensure ETag uniqueness per version by including Git commit hash. force
+    // browsers to reload dependent resources like app.js and zones.json even
+    // when index.html content hasn't actually changed between versions.
+    md5.add(String(__COMPILED_GIT_HASH__));
+
     md5.calculate();
 
     String expectedEtag;