implement Clean Architecture and user authentication

- Add LoopFit.Domain with entities (ApplicationUser, Workout, CalorieEntry, VitalData), enums (WorkoutType), and interfaces (IJwtTokenGenerator, IUserRepository, IWorkoutRepository)
- Add LoopFit.Application with auth DTOs, service interfaces, and DI setup
- Add LoopFit.Infrastructure with EF Core DbContext (PostgreSQL), ASP.NET Core Identity, JWT token generation, and repository implementations
- Add AuthController with register/login/me endpoints and JWT Bearer auth
- Add MAUI auth services (AuthService, TokenStorageService) and Login/Register Blazor pages

src/LoopFit.Infrastructure/Data/AppDbContext.cs

@@ -0,0 +1,18 @@
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore;
+using LoopFit.Domain.Entities;
+
+namespace LoopFit.Infrastructure.Data;
+
+public class AppDbContext : IdentityDbContext<IdentityUser>
+{
+    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
+    {
+    }
+
+    public DbSet<ApplicationUser> ApplicationUsers => Set<ApplicationUser>();
+    public DbSet<Workout> Workouts => Set<Workout>();
+    public DbSet<CalorieEntry> CalorieEntries => Set<CalorieEntry>();
+    public DbSet<VitalData> VitalData => Set<VitalData>();
+}