refactor: rename namespace to FiscalFlow and upgrade to .NET 10

- Rename InvoiceMaster.* to FiscalFlow.* namespace
- Upgrade from .NET 8 to .NET 10
- Update all NuGet packages to latest versions
- Update C# language version to 14.0

backend/src/FiscalFlow.Application/Commands/Auth/AuthCommands.cs

@@ -0,0 +1,35 @@
+using MediatR;
+
+namespace FiscalFlow.Application.Commands.Auth;
+
+public record RegisterCommand(
+    string Email,
+    string Password,
+    string? FullName) : IRequest<AuthResultDto>;
+
+public record LoginCommand(
+    string Email,
+    string Password) : IRequest<AuthResultDto>;
+
+public record RefreshTokenCommand(
+    string RefreshToken) : IRequest<TokenResultDto>;
+
+public record LogoutCommand(
+    Guid UserId) : IRequest<bool>;
+
+public record AuthResultDto(
+    bool Success,
+    string? ErrorMessage,
+    UserDto? User,
+    TokenResultDto? Tokens);
+
+public record TokenResultDto(
+    string AccessToken,
+    string RefreshToken,
+    int ExpiresIn);
+
+public record UserDto(
+    Guid Id,
+    string Email,
+    string? FullName,
+    DateTime CreatedAt);