feat(client): add CryptoService for login encryption

Implements ICryptoService for encrypting login credentials using RSA-OAEP.
Uses JavaScript interop with browser's native SubtleCrypto API instead of
Blazor.SubtleCrypto package (which only supports AES-GCM, not RSA-OAEP).

- ICryptoService interface in JdeScoping.Client.Services namespace
- CryptoService fetches server's public key once, caches it
- interop.js rsaEncrypt function for RSA-OAEP encryption via Web Crypto API

NEW/src/JdeScoping.Client/Services/ICryptoService.cs

@@ -0,0 +1,16 @@
+using JdeScoping.Core.Models.Auth;
+
+namespace JdeScoping.Client.Services;
+
+/// <summary>
+/// Service for encrypting data using server's RSA public key.
+/// </summary>
+public interface ICryptoService
+{
+    /// <summary>
+    /// Encrypts login credentials for transmission to server.
+    /// </summary>
+    /// <param name="model">Login credentials to encrypt</param>
+    /// <returns>Base64-encoded encrypted data</returns>
+    Task<string> EncryptLoginAsync(LoginModel model);
+}