Add keyboard shortcuts to Rapid Diffs merge request app

What does this MR do and why?

Binds MR keyboard shortcuts in Rapid Diffs: file navigation (]/j, [/k), commit navigation (c/x), and toggle reviewed (v).

Current file is determined by comparing getBoundingClientRect().top against the sticky header bottom edge (via getCoveringElementSync). No wrap-around at boundaries, matching legacy behavior.

Already working (no changes needed)

These shortcuts are handled by shared components already used in Rapid Diffs:

• f / shift+f (file browser) - FileBrowserToggle.vue
• ; / shift+; (expand/collapse all) - DiffAppControls.vue
• n / p (thread nav) - DiscussionNavigator
• b (copy branch) - ShortcutsIssuable
• mod+enter / shift+mod+enter (review/publish) - form-level

Deferred

• mod+p / t (Go to file) requires the file finder modal UI, tracked separately.

How to set up and validate locally

1. If you have not already opted in to Rapid Diffs, go to User Settings > Preferences > Diffs and enable Rapid Diffs
2. Navigate to any merge request with multiple changed files (e.g. http://localhost:3000/<group>/<project>/-/merge_requests/<iid>/diffs)
3. Press ] or j to jump to the next file; press [ or k to jump to the previous file
4. Press v to toggle the current file as reviewed (the checkbox should update and the file should collapse/expand accordingly)
5. Navigate to a commit view within the MR, then press c to go to the next commit and x to go to the previous commit
6. Verify that shortcuts do not fire when typing in a text input or textarea

References

Closes #504200 (closed)

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