Fix: docs(frontend): document the trading-terminal design system / style guide

[brite-side0]

Closes #1669

Summary

This is a documentation-only change. The design-system.md file does not yet exist, so it must be created. The README already has a Documentation section and a Further reading section; add the design-system link to the Documentation list near the top. Reference the pivot commits (bfe4ae3, 2494c91, e80a15b) explicitly in the principles section for traceability. If #1668 (a styling doc) is not yet merged, still link to ./docs/design-system.md since the file is being created in this change. Keep the tone consistent with the existing README's CSP section.

Changes

• Create frontend/docs/design-system.md with a 'Design principles' section explaining the trading-terminal direction and why the generic-AI-SaaS look was rejected, referencing pivot commits bfe4ae3, 2494c91, and e80a15b. (frontend/docs/design-system.md)
• Add a 'Do's and don'ts' section to design-system.md listing anti-patterns (no gradients, glow, glassmorphism, etc.) and the preferred conventions (flat surfaces, dense information, monospace numerics, high-contrast borders). (frontend/docs/design-system.md)
• Add a 'Tokens and variables' section documenting the shared palette, spacing scale, and CSS custom properties introduced by the redesign (or the conventions in use if no formal tokens exist). (frontend/docs/design-system.md)
• Add a 'Representative components' section pointing to components that exemplify the trading-terminal style (e.g. AppShell chrome, data tables, status indicators) with file paths. (frontend/docs/design-system.md)
• Add a link to ./docs/design-system.md in the Documentation list of frontend/README.md, alongside the existing styling and i18n links. (frontend/README.md)