This also adds support for ETag caching for the settings pages, specifically for the following pages ETag caching is now enabled:

• settings_wifi
• settings_leds
• settings_ui
• settings_sync
• settings_time
• settings_sec
• settings_dmx
• settings_um
• update
• settings_2D
• settingsCss
• welcome
• settings
• liveviewws2D
• liveview
• settingsCss
• favicon
• usermod
• index
• pixart
• pxmagic
• cpal

I also fixed some issues with the previous caching not being RFC 7232 compliant.
In Section 4.1 of RFC 7232 it is defined that a 304 (Not Modified) status code indicates that a conditional GET or HEAD request has been received and would have resulted in a 200 (OK) response. This is why I added a check if the status code was 200.
Another thing that the previous implementation did not do was that the server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. This is why I'm now calling the setStaticContentCacheHeaders function in the handleIfNoneMatchCacheHeader function.

To eliminate duplicate code, I created the function handleStaticContent. I added a comment that explains what this function does. Basically, now instead of having blocks like this:

server.on("/page.htm", HTTP_GET, [](AsyncWebServerRequest *request){
    if (handleFileRead(request, "/page.htm")) return;
    if (handleIfNoneMatchCacheHeader(request)) return;
    AsyncWebServerResponse *response = request->beginResponse_P(200, "text/html", PAGE_page, PAGE_page_L);
    response->addHeader(FPSTR(s_content_enc),"gzip");
    setStaticContentCacheHeaders(response);
    request->send(response);
});

We now have this:

server.on("/page.htm", HTTP_GET, [](AsyncWebServerRequest *request) {
    handleStaticContent(request, "/page.htm", 200, "text/html", PAGE_page, PAGE_page_L);
});

Which is much shorter. 😄

If you are wondering what the eTagSuffix is used for in the handleStaticContent function, it is currently not used, but I added it to have more control over caching. In the future this could be used if we want to use ETag caching for fxdata or effects.

I also fixed a small bug in cdata.js that caused favicon_length to have the wrong value.