feat(wall-tool): add Ctrl key to bypass grid snap for arbitrary wall lengths

[tryflowk]

Closes #242

Summary

• Adds Ctrl modifier key to the wall drawing tool that disables the 0.5m grid snap, allowing walls at any length (e.g. 1.70m, 2.35m)
• Shift continues to work as before — disabling angle snapping for non-45° walls
• Wall-join snapping (connecting to existing wall endpoints) still works in both free modes
• The two modifiers are fully independent and can be combined

Modifier key behaviour

Keys	Grid snap	Angle snap
(none)	0.5m grid	45°
Shift	0.5m grid	free
Ctrl	free	45°
Shift + Ctrl	free	free

Test plan

• Draw a wall without any modifier — snaps to 0.5m increments and 45° angles as before
• Hold Shift while drawing — angle is free, grid snap still active
• Hold Ctrl while drawing — can produce walls like 1.70m, 2.35m, etc.
• Hold Shift + Ctrl — fully free movement
• Verify wall-join snap (connecting to existing wall endpoints) still works with Ctrl held