Add friend library comparison and game comments features

- CompareController: side-by-side library diff with stats (common games, rating diff)
- GameCommentController + GameCommentPolicy: threaded comments with replies, auth-gated
- GameComment model with top-level scope and self-referential replies relation
- Migration: game_comments table with parent_id for threading
- games/show: Discussion section with Alpine.js (post, reply, delete)
- compare/show: tabbed view (in common, they have, you have) with game cards
- users/show: "Compare Libraries" button visible to other authenticated users

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Providers/AppServiceProvider.php

@@ -25,5 +25,6 @@ class AppServiceProvider extends ServiceProvider
         Gate::policy(UserGame::class, UserGamePolicy::class);
         Gate::policy(\App\Models\Review::class, \App\Policies\ReviewPolicy::class);
         Gate::policy(\App\Models\UserList::class, \App\Policies\ListPolicy::class);
+        Gate::policy(\App\Models\GameComment::class, \App\Policies\GameCommentPolicy::class);
     }
 }