Apply breakpoint-aware layout to issue_detail and mr_detail views:
- Issue detail: hide labels on Xs, hide assignees on Xs/Sm, skip milestone row on Xs
- MR detail: hide branch names and merge status on Xs/Sm
- Issue detail allocate_sections gives description 60% on wide (Lg+) vs 40% narrow
- Add responsive tests for both detail views
- Close bd-a6yb: all TUI screens now adapt to terminal width
760 lib tests pass, clippy clean.
026b3f0754