Added last modified header

[douxxtech]

Added the Last-Modified header, described here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Last-Modified

Also reorganized headers, that now look like this:

HTTP/1.0 200 OK
Date: Fri, 20 Mar 2026 20:17:50 GMT
Server: NASMServer/1.5
Last-Modified: Sat, 14 Mar 2026 19:47:08 GMT
Expires: Fri, 20 Mar 2026 20:17:50 GMT
Content-Type: text/html
Content-Length: 3501
Connection: close

[github-actions]

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.77ms   10.49ms 419.70ms   99.64%
    Req/Sec     1.75k   401.56     2.84k    73.38%
  139231 requests in 20.04s, 206.61MB read
  Socket errors: connect 0, read 28, write 0, timeout 0
Requests/sec:   6948.07
Transfer/sec:     10.31MB