feat: implement DiscordSessionBatchRenderer for Embed and Buttons

- Render SessionBatchViewModel into NetCord EmbedProperties + ActionRowProperties
- One embed per session with game title, Moscow date, players, capacity, waitlist, status
- Buttons map AvailableAction to ButtonProperties with platform-neutral custom IDs
- Cancelled sessions get embed but no action row
- Full sessions trigger waitlist button label
- 7 tests covering open/full/waitlist/cancelled/reschedule states

Closes #27

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

src/GmRelay.Shared/Rendering/DiscordSessionBatchRenderer.cs

@@ -1,13 +0,0 @@
-namespace GmRelay.Shared.Rendering;
-
-/// <summary>
-/// Заглушка для Discord-рендерера.
-/// Реальная реализация будет добавлена в проект GmRelay.DiscordBot (issue #26).
-/// </summary>
-public static class DiscordSessionBatchRenderer
-{
-    public static object Render(SessionBatchViewModel view)
-    {
-        throw new NotImplementedException("Discord renderer will be implemented in issue #26.");
-    }
-}