hotfix: fix Blazor circuit crash on Discord link + add missing avatar_url column

- Replace @onclick button with plain <a href="/auth/discord"> to avoid
circuit disconnect from forceLoad navigation during event handlers.
- Add query param handling (?linked, ?link_error) in Profile.razor for
Discord callback feedback.
- Add V021 migration: ALTER TABLE players ADD COLUMN avatar_url.

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

src/GmRelay.Bot/Migrations/V021__add_avatar_url.sql

@@ -0,0 +1,8 @@
+-- =============================================================
+-- V021: Add avatar_url column to players table
+-- =============================================================
+-- Scope: Support storing avatar URLs for Discord and other platforms.
+-- =============================================================
+
+ALTER TABLE players
+    ADD COLUMN avatar_url VARCHAR(500);

src/GmRelay.Web/Components/Pages/Profile.razor

@@ -3,7 +3,6 @@
 @using Microsoft.AspNetCore.Components.Authorization
 @attribute [Authorize]
 @inject ISessionStore SessionStore
-@inject NavigationManager Navigation
 
 <PageTitle>Профиль — GM-Relay</PageTitle>
 
@@ -54,9 +53,9 @@
         <h2 class="section-title">Добавить аккаунт</h2>
         @if (!HasLinkedPlatform("Discord"))
         {
-            <button class="btn btn-primary" @onclick="LinkDiscord">
+            <a href="/auth/discord" class="btn btn-primary">
                 Привязать Discord
-            </button>
+            </a>
         }
         else
         {
@@ -86,6 +85,12 @@
     [CascadingParameter]
     private Task<AuthenticationState>? AuthenticationStateTask { get; set; }
 
+    [SupplyParameterFromQuery]
+    public string? Linked { get; set; }
+
+    [SupplyParameterFromQuery(Name = "link_error")]
+    public string? LinkError { get; set; }
+
     protected override async Task OnInitializedAsync()
     {
         if (AuthenticationStateTask is not null)
@@ -99,6 +104,16 @@
             }
         }
 
+        if (!string.IsNullOrWhiteSpace(Linked))
+        {
+            successMessage = $"{Linked} аккаунт успешно привязан!";
+        }
+
+        if (!string.IsNullOrWhiteSpace(LinkError))
+        {
+            errorMessage = $"Ошибка привязки: {Uri.UnescapeDataString(LinkError)}";
+        }
+
         await LoadIdentities();
     }
 
@@ -157,9 +172,4 @@
             isUnlinking = false;
         }
     }
-
-    private void LinkDiscord()
-    {
-        Navigation.NavigateTo("/auth/discord", forceLoad: true);
-    }
 }