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

frontend/src/app/globals.css

@@ -1,26 +1,11 @@
 @import "tailwindcss";
 
-:root {
-  --background: #ffffff;
-  --foreground: #171717;
-}
-
 @theme inline {
-  --color-background: var(--background);
-  --color-foreground: var(--foreground);
   --font-sans: var(--font-geist-sans);
   --font-mono: var(--font-geist-mono);
 }
 
-@media (prefers-color-scheme: dark) {
-  :root {
-    --background: #0a0a0a;
-    --foreground: #ededed;
-  }
-}
-
 body {
-  background: var(--background);
-  color: var(--foreground);
-  font-family: Arial, Helvetica, sans-serif;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
 }