Add notifications API for PostIt home page

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Api.Client/NotificationsApiClient.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+using Yavsc.Abstract.Models.Messaging;
+
+namespace Yavsc.Api.Client;
+
+/// <summary>
+/// Thin HTTP client for the notifications exposed to PostIt on the home page.
+/// </summary>
+public sealed class NotificationsApiClient
+{
+    private const string PathPrefix = Constants.NotificationsPath;
+
+    private readonly IYavscApiClient _api;
+    private readonly Func<string> _businessBaseAddress;
+
+    public NotificationsApiClient(IYavscApiClient api, string businessBaseAddress)
+        : this(api, () => businessBaseAddress)
+    {
+    }
+
+    public NotificationsApiClient(IYavscApiClient api, Func<string> businessBaseAddress)
+    {
+        _api = api ?? throw new ArgumentNullException(nameof(api));
+        _businessBaseAddress = businessBaseAddress ?? throw new ArgumentNullException(nameof(businessBaseAddress));
+        _ = ResolveBusinessBaseAddress();
+    }
+
+    public Task<List<Notification>> GetCurrentAsync(CancellationToken ct = default)
+    {
+        return _api.CallAsync<List<Notification>>(
+            HttpMethod.Get,
+            Absolute($"{PathPrefix}/{Constants.NotificationsCurrentUserRoute}"),
+            ct: ct);
+    }
+
+    public Task<List<Notification>> GetAllAsync(CancellationToken ct = default)
+    {
+        return _api.CallAsync<List<Notification>>(
+            HttpMethod.Get,
+            Absolute(PathPrefix),
+            ct: ct);
+    }
+
+    private string Absolute(string relativePath) => new Uri(ResolveBusinessBaseAddress(), relativePath).ToString();
+
+    private Uri ResolveBusinessBaseAddress()
+    {
+        var raw = _businessBaseAddress();
+        if (string.IsNullOrWhiteSpace(raw))
+            throw new InvalidOperationException("Business base address is required.");
+
+        return new Uri(raw, UriKind.Absolute);
+    }
+}