feat: implement Tauri commands for jj operations using jj-lib (TAT-34)
Add Tauri backend commands for Jujutsu repository operations:
- find_repository: locate jj workspace root
- get_revisions: fetch revision log via revset API
- get_status: get working copy status via tree diff
- get_file_diff: compute file diffs using similar crate
Includes TypeScript types and invoke wrappers for frontend.
Issue: TAT-34