Rewrite README with detailed portal and accessibility documentation

[FolatheDuckofDuckingburg]

Re-wrote README.md to comprehensively document PenPal's three user portals (/dashboard, /parent, /educator), accessibility design system, localStorage persistence keys, directory structure, accurate TypeScript code examples, and local setup instructions.

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PR created automatically by Jules for task 14422653438702590284 started by @FolatheDuckofDuckingburg

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