itag-18 fallback videos (low res 360p) fail to seek later in videos

### Official Instance - [ ] The bug is reproducible on the [official hosted instance](http://piped.video/), or is API-related. ### Describe the bug In short it's a piped-proxy issue with how it interprets the length of the video in bytes for the headers. I have solved this on my fork's image: logicalkarma/piped-proxy. Write-up from Claude: Since handle_range_response_correction uses the upstream response's Content-Length as total_size, any range request with a non-zero start byte produces a malformed Content-Range header. The upstream Content-Length reflects the size of the returned chunk, not the full file, so the synthesized header has end < start and the wrong denominator. Curl ignores the header mismatch, and Librewolf typically dodges the bug because its eager-prefetch behavior issues a single open-ended range from byte 0 and never seeks past the buffer. Vanilla Firefox and Chromium both hit it on seek-past-buffer in itag-18 (360p progressive) playback, presenting as an indefinite "loading" spinner; in Chromium, DevTools shows the response aborting after ~700 bytes. The fix is to use the clen query parameter (canonical full file size) instead of the upstream Content-Length. ### To Reproduce 1. Find a video that fallbacks to itag-18 (no configurable resolution - 360p) 2. Go somewhere towards the end of the video. 3. Player loads indefinitely ### Expected behavior 1. Find a video that fallbacks to itag-18 (no configurable resolution - 360p) 2. Go somewhere towards the end of the video. 3. Player moves to the expected time and plays ### Logs/Errors _No response_ ### Browser, and OS with Version. Anything that does not cache the entire video. All browsers except some privacy centric ones which avoid this bug unintentionally (Ironwolf, Librewolf). ### Additional context _No response_