docs: update website for 0.5.0 design policy

[whitelonng]

What changed

• add a high-contrast 0.5.0 design-policy section to both landing pages
• explain the 2–3 direction decision rule, preserve/refine/experimental presets, and scope guardrails
• add a complete bilingual mancode design documentation section and correct legacy-only --style guidance
• replace the theme glyph with inline Lucide sun and moon icons
• update website contract tests for the new content and section order

Why

The public website showed version 0.5.0 but did not explain the release's primary UI-design capability. Users could not discover how direction selection works, how to configure the policy, or which safety boundaries remain in force.

User impact

Visitors can now understand and configure the design policy from the official site without consulting repository internals. The original opening section and established brand system remain intact.

Validation

• npm run lint
• npm run typecheck
• npm run build
• npm test -- --silent (122 files, 858 tests)
• website contract tests (8 passed)
• Playwright headless checks at 1440px and 390px for both languages, both docs layouts, theme icons, and horizontal overflow
• git diff --check