feat: add frontend auth pages, app layout, and Supabase integration (Phase 1E)
- Auth pages: login (password + magic link), register (with firm name), callback
- Supabase client setup: browser client, server client, middleware for session refresh
- App layout: sidebar (Dashboard, Akten, Fristen, Termine, AI Analyse, Einstellungen),
header with user info and tenant switcher
- Shared: API client with auth headers, TypeScript types matching Go models,
QueryClientProvider + Toaster providers
- Dependencies: @supabase/supabase-js, @supabase/ssr, @tanstack/react-query,
lucide-react, date-fns, sonner
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