gui: unify design system and typography

[Ingwannu]

What changed

• introduce role-based typography, spacing, radius, motion, icon, and control tokens for the GUI
• normalize page-level typography across Dashboard, Providers, Models, Logs, Debug, Usage, Codex Auth, Subagents, and Claude Code
• improve dark-mode enabled-toggle contrast and add separation between sidebar navigation items
• add an opt-in Vite same-origin proxy for integrated local GUI testing
• document foundations, components, contribution rules, and architecture decisions under docs/design-system/

Why

The GUI had thirteen near-duplicate font sizes, page-local weights/radii, and two switch implementations with inconsistent dark-mode contrast. Adjacent sidebar hover/active surfaces also visually merged. Centralizing these decisions gives every page the same hierarchy and makes future light/dark changes maintainable.

User impact

• clearer enabled-toggle state in dark mode
• more distinct sidebar hover and active states
• consistent UI, body, label, code, and heading typography across the full GUI
• documented rules for future GUI changes

Validation

• cd gui && bun run lint (0 errors; 2 existing TanStack Virtual compiler warnings)
• cd gui && bun run build
• Playwright Chromium QA across all 10 navigation destinations
• dark, light, and 390x844 mobile drawer visual checks
• no console errors or framework overlay
• local Vite GUI, /healthz, and /api/settings all returned HTTP 200 through the opt-in proxy
• token audits confirmed no direct numeric font size/weight/line-height or border-radius values remain in GUI CSS/TSX