slightly reduce flickering during file access

it seems that reading/writing "larger" files from LittleFS causes LED flickering.

This change adds a simple cache for "file not found" results, so that repeated file.exists() calls are avoided.

wled00/wled_server.cpp

@@ -58,6 +58,7 @@ void handleUpload(AsyncWebServerRequest *request, const String& filename, size_t
     request->_tempFile.close();
     USER_PRINT(F("File uploaded: "));  // WLEDMM
     USER_PRINTLN(filename);            // WLEDMM
+    invalidateFileNameCache();         // WLEDMM
     if (filename.equalsIgnoreCase("/cfg.json") || filename.equalsIgnoreCase("cfg.json")) { // WLEDMM
       request->send(200, "text/plain", F("Configuration restore successful.\nRebooting..."));
       doReboot = true;