Add radius slider to customize corner radius

[squain]

Addresses issue #36 where user wanted to disables fillets.

1. Added MIN_RADIUS = 0 and MAX_RADIUS = 20 constants
2. Created a new radiusControl range slider labeled "Corner Radius"
3. Added the control to the inputs object
4. Wired up event listeners to sync the UI with modelDimensions.radius
5. Added radius input to the focus-select-all behavior

How it works:

• Set Corner Radius to 0 → sharp 90° corners
• Set Corner Radius to 6 (default) → current rounded appearance
• Range is 0-20mm

I appreciate it complicates the minimalist interface slightly..

[CasperVerswijvelt]

Since the width/height parameters determine the inner dimensions, it might make sense to let the radius parameter determine the inner radius instead of current outer radius, but then you won't be able to disable the outer fillet though (0mm radius with 2mm wall thickness results in 2 mm outer radius)...