Add OpenClaw integration, preview UI polish, and README updates

[AIPMAndy]

Motivation

• Provide a lightweight host-to-PageSkill orchestration protocol so external pages can programmatically control markdown, template, view mode and exports.
• Improve preview visuals and app shell to make mobile/tablet/desktop previews clearer and more branded.
• Document current project health, recommended next steps and OpenClaw usage in both Chinese and English READMEs.

Description

• Add src/utils/openclaw.ts which defines OpenClawCommand/OpenClawAck, URL param parsing (parseOpenClawURLParams), message validation (isOpenClawMessage), template resolution (resolveTemplateById) and ack posting (postOpenClawAck).
• Integrate OpenClaw in src/App.tsx by adding dispatchOpenClawCommand, parsing URL params on load, adding a message listener for incoming commands, and exposing a host API on window.PageSkillOpenClaw with dispatch and getState.
• Enhance preview component src/components/Preview/Preview.tsx with adjusted mobile/tablet widths, labeled preview header, icons, refined container styles and preserved animation.
• Minor header branding update in src/components/Header/Header.tsx to rename the app to PageSkill and add an OpenClaw Ready badge.
• Add visual polish CSS in src/index.css including .app-shell background and .preview-surface grid to improve surface aesthetics.
• Update README.md and README_EN.md with a project health check, recommended next steps, a growth playbook, OpenClaw usage examples (URL params and postMessage) and common commands.

Testing

• Ran type/lint check via npm run lint and it passed.
• Built a production bundle via npm run build and the build succeeded.
• lint:eslint currently fails because the repository does not include an ESLint v9 flat eslint.config.*, and this is noted in the README as the recommended next step.

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Codex Task