feat: add video quality selection for yt-dlp captures (#17)

- ytdlp::download() accepts quality: Option<&str>; quality_format()
  maps best/1080p/720p/480p/360p to yt-dlp -f format strings
- perform_capture() threads quality through to the downloader
- CaptureBody gains optional quality field; capture_handler validates
  it against the allowlist (400 on unknown values) before spawning
- CLI passes None (preserves existing best-quality behaviour)
- Frontend: isVideoSource() mirrors determine_source() exactly —
  shows quality picker only for yt-dlp-backed sources, excludes
  playlist/channel shorthands and tweet/thread paths
- submitCapture(archiveId, locator, quality) sends quality in POST body
- CSS: .capture-quality styles the inline select to fit the capture row
- Tests: quality_format unit tests in ytdlp.rs; two new route tests
  (valid quality accepted, invalid quality rejected with 400)
- Docs: video quality section added under Supported Platforms

crates/archivr-cli/src/main.rs

@@ -63,7 +63,7 @@ fn main() -> Result<()> {
                 }
             };
             let archive_paths = archive::read_archive_paths(&archive_path)?;
-            let result = archivr_core::capture::perform_capture(&archive_paths, path, None)?;
+            let result = archivr_core::capture::perform_capture(&archive_paths, path, None, None)?;
             println!("Archived: run {}", result.run_uid);
             Ok(())
         }