feat(web): add appearance color schemes

[sheepbox8646]

Summary

• add an Appearance settings page for language, theme mode, and color scheme selection
• introduce scheme-aware global color tokens and palette templates
• replace scattered hardcoded UI colors with semantic tokens across the web frontend
• soften default control, badge, chat bubble, and access selection colors for light/dark themes

Verification

• pnpm --filter @memohai/web build
• git diff --check
• pre-commit Go tests and frontend eslint hooks