Upgrade platform README

[computersciencefreshmen]

What changed

• rewrote the Chinese and English READMEs with a polished product-style hero, navigation, technology badges, and canonical repository links
• grouped features by student, teacher, administrator, and shared platform capabilities
• added an architecture diagram, role entry points, capability-maturity matrix, runtime configuration, quality gates, deployment notes, and roadmap
• documented the three-repository consolidation and the standard merge-commit contribution workflow

Why

The previous documentation explained setup but did not communicate the platform's product scope, engineering boundaries, or current maturity clearly. The new version follows the information architecture of a professional showcase README while avoiding unsupported backend, security, deployment, or licensing claims.

Impact

New contributors and reviewers can understand what is implemented, which features depend on external services, how to run the application, and where the project is heading without reading the full source tree.

Verification

• Prettier formatting for both README files
• relative README links and local image paths validated
• git diff --check
• pnpm check (ESLint, Prettier, production build)