Design system: DESIGN.md tokens + dark/light palette-only theming + rebuilt UI primitives

[ajianaz]

Requires uteke: v0.16.0+ (no API change) · Epic: #289

What

• DESIGN.md + src/app.css: token set in OKLCH — color (bg/surface/border/text 3 levels, primary teal, amber semantic-only, success/danger), 3-step radius (12/16/8 card/CTA/pill; sm 4-6), spacing scale, elevation, motion (150ms ease-out, press scale 0.98), type scale (sans UI + mono for IDs/numbers/dates, tabular figures for dynamic numbers).
• Theme = palette-only switch (decision 2026-09-06): dark default = editor-grade (Linear/Raycast school), light = warm paper palette from variant B (same type/spacing). No per-theme typography.
• Rebuild src/lib/ui/* (Button, Input, Select, Card, Badge, Modal, Toast, EmptyState, Skeleton, Kbd, Tooltip, ConfirmDialog) consuming tokens only; color lock: teal = brand action, amber = semantic score/warning.
• Theme store (localStorage + prefers-color-scheme default) + ThemeToggle (rail + Settings→Appearance).

Why

Vision audit found contrast failures (WCAG AA) and ad-hoc styling; uteke-mobile already proved the 3-token radius + color-lock discipline. Palette-only keeps one identity across themes.

Testing

• Contrast script: all text-role token pairs ≥ WCAG AA in both themes.
• grep gate: no raw hex/oklch literals outside tokens file in components.
• Vitest: theme store toggle + persistence; bun run test + build green.
• Visual: journey screenshots dark+light reviewed by vision model (0 HIGH).