Add core models, DTOs, and services for EchoHub chat application

- Created User, Channel, Message, and ServerInfo models with necessary properties.
- Added DTOs for user profiles, server information, and message handling.
- Implemented JWT authentication service for user login and token generation.
- Developed Entity Framework Core DbContext for database interactions.
- Introduced SignalR ChatHub for real-time messaging and presence tracking.
- Implemented file storage and image to ASCII conversion services.
- Set up CORS and middleware for API endpoints.
- Created launch settings and development configuration for server and web projects.

src/EchoHub.Server/Auth/JwtTokenService.cs

@@ -0,0 +1,40 @@
+using System.IdentityModel.Tokens.Jwt;
+using System.Security.Claims;
+using System.Text;
+using EchoHub.Core.Models;
+using Microsoft.IdentityModel.Tokens;
+
+namespace EchoHub.Server.Auth;
+
+public class JwtTokenService(IConfiguration configuration)
+{
+    private readonly string _secret = configuration["Jwt:Secret"]
+        ?? throw new InvalidOperationException("Jwt:Secret is not configured.");
+    private readonly string _issuer = configuration["Jwt:Issuer"]
+        ?? throw new InvalidOperationException("Jwt:Issuer is not configured.");
+    private readonly string _audience = configuration["Jwt:Audience"]
+        ?? throw new InvalidOperationException("Jwt:Audience is not configured.");
+
+    public string GenerateToken(User user)
+    {
+        var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_secret));
+        var credentials = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
+
+        Claim[] claims =
+        [
+            new(JwtRegisteredClaimNames.Sub, user.Id.ToString()),
+            new("username", user.Username),
+            new("display_name", user.DisplayName ?? user.Username),
+            new(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
+        ];
+
+        var token = new JwtSecurityToken(
+            issuer: _issuer,
+            audience: _audience,
+            claims: claims,
+            expires: DateTime.UtcNow.AddDays(7),
+            signingCredentials: credentials);
+
+        return new JwtSecurityTokenHandler().WriteToken(token);
+    }
+}