feat: implement auth, storage, caching, and export updates

[Tinna23]

Summary

• Add a dedicated admin root layout that wraps every admin route with the toast context, admin shell, and no-index metadata.
• Provide a thin /admin client redirect alongside an animated admin error boundary that surfaces actions plus development details.
• Implement the admin change-password form with zod/react-hook-form validation, show/hide controls, helper copy, and a security tips panel.

Testing

• Not run (npm install --no-audit --no-fund hung in this environment before linting could run).

Closes #527
Closes #529
Closes #530
Closes #533

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