Support Caps Lock and Spacebar remapping

[Ashesh3]

Summary

• allow Caps Lock and Space to be captured as remappable keys
• display Space as Spacebar and CapsLock as Caps Lock across key prompts and key previews
• keep longer key labels inside keycap boxes
• fix the remap flow so Space/Caps Lock register with one keypress
• replace checkbox SVG data URIs with CSS shapes so production CI builds do not fail on CSS optimizer warnings

Test

• CI=true npm run build
• Browser-tested local remapping: one press changes Q to Spacebar and W to Caps Lock, with labels fitting in the options key boxes.

Note: this PR intentionally excludes README.md and .gitignore changes.

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