feat: User-Management mit Token-basierter Authentifizierung

Backend:
- UserRecord Model + Users-Tabelle in DB
- AuthService: SHA256-Passwort-Hashing, Token-Verwaltung (ConcurrentDictionary)
- AuthMiddleware: Schützt alle /api/-Routes (außer /api/auth/login)
- AuthEndpoint: Login, Logout, Me
- Default-User admin/admin wird automatisch erstellt

Frontend:
- Login-Page mit Formular (Enter-Key-Support)
- Globaler Fetch-Interceptor: 401 → automatisch Login-Redirect
- Auth-Check beim App-Start und bei Hash-Änderungen
- Logout-Link im Dashboard-Header
- Login-CSS: Zentrierte Login-Box

Program.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using Gott.Endpoints;
+using Gott.Middleware;
 using Gott.Services;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
@@ -10,17 +11,24 @@ builder.Services.AddHttpClient();
 
 var dataDir = Path.Combine(AppContext.BaseDirectory, "data");
 
+var authService = new AuthService();
+var dbService = new DatabaseService(Path.Combine(dataDir, "gott.db"));
+dbService.EnsureDefaultUser(authService);
+
+builder.Services.AddSingleton(authService);
+builder.Services.AddSingleton(dbService);
 builder.Services.AddSingleton(new ToolService());
 builder.Services.AddSingleton(new ImageService());
 builder.Services.AddSingleton(new StreamingTracker());
 builder.Services.AddSingleton(new MemoryService(dataDir));
 builder.Services.AddSingleton(new SkillService(dataDir));
-builder.Services.AddSingleton(new DatabaseService(Path.Combine(dataDir, "gott.db")));
 
 var app = builder.Build();
 app.UseDefaultFiles();
 app.UseStaticFiles();
+app.UseMiddleware<AuthMiddleware>();
 
+app.MapAuthEndpoints();
 app.MapChatEndpoint();
 app.MapHistoryEndpoint();
 app.MapSessionEndpoints();