Add asset management system

- Created Assets.cshtml and Assets.cshtml.cs for admin panel
- Upload assets with MD5/SHA256 hash calculation
- Generate asset manifests in RR3 format (tab-separated)
- Integrated with Nimble SDK asset download system
- Updated GameAsset model with IsRequired, UploadedAt, Description
- Added navigation link in _Layout.cshtml
- Supports categories: base, cars, tracks, audio, textures, UI, DLC
- Asset download endpoint at /content/api/{assetPath}
- Manifest endpoint at /content/api/manifest

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

RR3CommunityServer/Pages/_Layout.cshtml

@@ -103,6 +103,16 @@
                             <i class="bi bi-cart"></i> Purchases
                         </a>
                     </li>
+                    <li class="nav-item">
+                        <a class="nav-link" href="/assets">
+                            <i class="bi bi-box-seam"></i> Assets
+                        </a>
+                    </li>
+                    <li class="nav-item">
+                        <a class="nav-link" href="/settings">
+                            <i class="bi bi-gear"></i> Settings
+                        </a>
+                    </li>
                     <li class="nav-item">
                         <a class="nav-link" href="/swagger" target="_blank">
                             <i class="bi bi-code-slash"></i> API