feat(ui): unassigned album selector in PersonForm, Add Albums button in CosplayerDetail

MilkStream.Client/Services/AlbumService.cs

@@ -46,6 +46,30 @@ public sealed class AlbumService(
         return null;
     }
 
+    /// <summary>
+    /// Gets all albums that have no person assigned.
+    /// </summary>
+    /// <returns>A list of unassigned album previews, or null if the request failed.</returns>
+    public async Task<List<AlbumPreviewDto>?> GetUnassignedAsync() {
+        var response = await SendWithRefreshAsync(() => Client.GetAsync("/api/album/unassigned"));
+
+        if (response.IsSuccessStatusCode)
+            return await response.Content.ReadFromJsonAsync<List<AlbumPreviewDto>>();
+
+        return null;
+    }
+
+    /// <summary>
+    /// Assigns a person to an album.
+    /// </summary>
+    /// <param name="albumId">The album ID.</param>
+    /// <param name="personId">The person ID to assign.</param>
+    public async Task AssignPersonAsync(Guid albumId, Guid personId) {
+        var dto = new AlbumUpdateDto { Person = personId };
+        var response = await Client.PostAsJsonAsync($"/api/album/{albumId}", dto);
+        response.EnsureSuccessStatusCode();
+    }
+
     /// <summary>
     /// Creates a new album.
     /// </summary>