Fix workspace shell layout and admin sidebar mapping

[Cataldir]

Summary

• fixes admin sidebar route matching so nested configuration routes no longer mark multiple items active
• replaces the misleading Users admin entry with Avatar cases and aligns admin workspace copy with the real surfaces
• tightens workspace shell spacing, sidebar width, header offsets, and global body typography/color defaults
• adds route-shell coverage for admin active nav states and mobile/tablet reflow

Validation

• pnpm --dir frontend typecheck
• pnpm --dir frontend lint
• pnpm --dir frontend exec playwright test e2e/route-shells.spec.ts
• git diff --check