fix(checkbox): remove unsupported is prop and clean up Storybook

[ShubhamS2005]

Problem

The CheckBox component no longer supports the is prop (was previously inherited from Box).
Storybook still had a control for is, causing TypeScript errors and Storybook crashes.

Steps to Reproduce

1. Open Storybook for Inputs/CheckBox
2. Try to use the is control in Storybook
3. Observe TypeScript error:
   type 'string' is not assignable to type 'ElementType | undefined'
4. Component still works, but Storybook shows errors and crashes when is is selected.

Solution

• Removed is control from Storybook for CheckBox
• Kept CheckBoxProps clean: Omit<BoxProps, 'is'>
• Checkbox functionality remains unchanged
• Storybook runs without error

Expected Behavior

• No TypeScript errors
• Storybook loads cleanly
• CheckBox works normally

Actual Behavior

• TypeScript error when passing is
• Storybook crash or object showing instead of proper element

Backward-compatible fix
Tested manually in Storybook:
Closes #1904

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