Move Window Up shortcut does not switch to monitor above in vertically stacked monitor layouts

**Describe the bug**
- When monitors are stacked vertically (one above another), moving a window with keyboard shortcuts works only in one direction: I can move a window down to the other monitor, but I cannot move it back up.
- `Move Window Up` is consumed as a local maximize action instead of transitioning to the monitor above.

**To Reproduce**
Steps to reproduce the behavior:
1. Arrange two monitors vertically in GNOME (monitor B above monitor A).
2. Focus a normal window on the above monitor (A).
3. Trigger the `Move Window Down` shortcut (works as expected).
4. Trigger the `Move Window Up` shortcut.
5. See error: the window does not move to the monitor above.

**Information (please complete the following):**
- Tiling Shell version: v17.3
- GNOME version: 46.0

**Additional context**
Root cause appears to be in keyboard move handling:
- `UP` can be handled as maximize on the current monitor before monitor transition fallback is attempted.
- This makes `DOWN` work while `UP` fails in vertical monitor layouts.

Proposed behavior:
- Keep backward-compatible default (`UP` maximizes).
- Add a setting to allow `UP` to prioritize moving to a monitor above when one exists.