feat(web): add Discord login button, platform indicator, and CSS styles

- Discord login button on /login with brand colors
- NavMenu shows user avatar (Discord) and platform label
- CSS: login-divider, login-btn-discord, nav-user-info, nav-user-platform
- NavMenu version bumped to v2.8.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/Components/Layout/NavMenu.razor

@@ -39,9 +39,19 @@
             <div class="nav-footer">
                 <div class="nav-user">
                     <div class="nav-user-avatar">
-                        @(context.User.Identity?.Name?.Substring(0, 1).ToUpper() ?? "?")
+                        @if (!string.IsNullOrWhiteSpace(context.User.GetAvatarUrl()))
+                        {
+                            <img src="@context.User.GetAvatarUrl()" alt="" />
+                        }
+                        else
+                        {
+                            @(context.User.Identity?.Name?.Substring(0, 1).ToUpper() ?? "?")
+                        }
+                    </div>
+                    <div class="nav-user-info">
+                        <span class="nav-user-name">@context.User.Identity?.Name</span>
+                        <span class="nav-user-platform">@GetPlatformLabel(context.User)</span>
                     </div>
-                    <span class="nav-user-name">@context.User.Identity?.Name</span>
                 </div>
 
                 <form action="/auth/logout" method="post">
@@ -56,7 +66,7 @@
                     </button>
                 </form>
 
-                <div class="nav-version">v2.7.2</div>
+                <div class="nav-version">v2.8.0</div>
             </div>
         </Authorized>
         <NotAuthorized>
@@ -79,4 +89,7 @@
 
     private void ToggleMenu() => isOpen = !isOpen;
     private void CloseMenu() => isOpen = false;
+
+    private static string GetPlatformLabel(System.Security.Claims.ClaimsPrincipal user) =>
+        user.TryGetDiscordId(out _) ? "Discord" : "Telegram";
 }