feat: resumable downloads with cache, abort dialog, and Range-error recovery

[AvengerAnubis]

This PR adds three related features to the download system:

1. Resumable downloads with partial file support

• introduced ResumingFileSink — a new sink that downloads to a .part file,
  sets Range and Accept-Ranges headers so interrupted downloads can resume
• added DownloadCache — stores completed downloads keyed by URL hash,
  promotePart() copies the final file to the cache directory on success,
  serveFromCache() restores it on re-download with validator checksum verification
• cache hits skip the download entirely; if validators fail, it falls through to
  a fresh download

2. Abort confirmation dialog

• when the user clicks "Abort" during a download, a QMessageBox asks whether
  to delete the cached .part file (Yes/No)
• NetJob::deleteCachedDownloads() cleans up all cached artifacts for the job

3. Range-error recovery (404/416 on resumed downloads)

• Some CDNs (CloudFront + S3) return 404 or 416 when a Range request
  refers to a stale or invalid offset
• NetRequest::downloadError() now detects these status codes on resumable
  downloads, deletes the stale .part via deleteCache(), and retries once
  from scratch without Range

Files changed

• launcher/net/ResumingFileSink.{h,cpp} — new sink, cache-hit validation,
  hasUsedRange() detection, deleteCache()
• launcher/net/DownloadCache.{h,cpp} — cache storage, promotePart(),
  serveFromCache(), metadata management
• launcher/net/NetRequest.{h,cpp} — deleteCachedDownload(), 404/416 retry
• launcher/net/NetJob.{h,cpp} — deleteCachedDownloads() iterates all subtasks
• launcher/net/Download.{h,cpp} — wire up cache in existing downloads
• launcher/net/ApiDownload.cpp — pass cache through
• launcher/ui/dialogs/ProgressDialog.cpp — abort confirmation dialog
• launcher/ui/pages/global/LauncherPage.{cpp,ui} — cache settings UI
• launcher/Application.{h,cpp}, launcher/CMakeLists.txt — wiring
• .gitignore — ignore local project files

[herakl1209]

Question, why are you submitting this here? This is just a fork of Prism Launcher. Usually, it doesn't add "big" features on it's own. Maybe you should submit this at the Prism Launcher PRs?

[AvengerAnubis]

I'll try it maybe one day, gonna need to fork the original though. Forked this launcher for my own sake, and added this feature cause I wasn't able to download any large modpacks, poor internet connection hits hard, and thought why not pull request it.
Thanks for the suggestion, have a nice day

[AvengerAnubis]

made a pull request to PrismLauncher, may as well close this one