chore: Remove deprecated and dead code

[Nicolapps]

Problem: The codebase contains several deprecated or dead code artifacts:

• MessageInput.tsx is marked @deprecated but still present
• Instruction.tsx has a dead CSS class (overridden on the same line)
• globals.css has duplicated @layer base blocks and redundant body styles
• Dark mode CSS variables are fully defined but never used
• attempts.ts contains a @todo Remove this logic comment on legacy research group code

Proposed Solution

• Delete MessageInput.tsx (verify no remaining imports)
• Clean up Instruction.tsx redundant class
• Consolidate duplicated CSS layers in globals.css
• Remove or gate the unused dark mode CSS
• Address the TODO in attempts.ts