feat: support drag-and-drop pairing of renamed files in webdiff

[JonahSussman]

Implements #335.

When comparing directories, renamed files appear as separate "deleted" and "added" entries with no way to compare them. This PR adds drag-and-drop support in the webdiff directory list view so users can manually pair renamed files for comparison.

Before

Renamed files (OldName.java -> NewName.java) show up as unrelated deleted/added entries:

After

Drag a file from the Deleted section onto a file in the Added section (or vice versa) to pair them. Paired files appear in the Modified section as OldName.java -> NewName.java with all diff viewers available, plus an "unpair" button to reverse the operation:

Clicking any diff viewer on the paired entry works as expected:

Changes

• DirectoryComparator - Added pairFiles() and unpairFiles() methods
• WebDiff - Added POST /pair-files and POST /unpair-files endpoints
• DirectoryDiffView - Drag-and-drop attributes on deleted/added rows, rename display (old → new), unpair button; also fixed added files being relativized against the wrong root directory
• dragdrop.js (new) - Client-side HTML5 drag-and-drop handling
• TestDirectoryComparator - Added tests for pair/unpair logic and invalid argument handling

[jrfaller]

Hi! Thanks a lot for the PR. The patch looks good to me. I will see what the pipeline says! Cheers.

[JonahSussman]

@jrfaller No problem, glad I could help out!