fix: improve checkbox contrast in folder view

[cqnykamp]

Summary

• Fixes insufficient contrast on checkboxes in My Activities folder #2891: checkboxes in the My Activities folder view were very hard to see
• Overrides Chakra UI's default Checkbox border color from gray.200 (very light, #E2E8F0) to gray.500 (#718096), which provides adequate contrast on white backgrounds
• The _hover state also gets a slightly darker gray.600 border for visual feedback
• This applies to all Chakra UI <Checkbox> instances across the app (folder view, filter panel, editor, etc.)

Test plan

• Visit My Activities folder — checkboxes on each row should now have a clearly visible gray border
• Verify hover state shows slightly darker border
• Verify checked state is unchanged (blue fill, white checkmark)
• Verify disabled checkboxes still render correctly
• Run existing Cypress component tests (npm run test:all --workspace @doenet-tools/app)

https://claude.ai/code/session_01KxyYxeTTrz3tbA9abPP81T

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