Apply DESIGN.md visual system

[peterabcd]

Summary

• add DESIGN.md-aligned app chrome, theme tokens, and shared UI component styling
• restyle auth, home, sessions, runner, and admin pages around Apple-style parchment/canvas surfaces and Action Blue interactions
• keep scope to visual system changes; no new product feature added

Verification

• pnpm exec vitest run test/form-and-filter.test.tsx test/session-filters.test.ts
• pnpm run typecheck
• pnpm run lint
• pnpm run test
• pnpm run build
• git diff --check