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/Models/Game.php

@@ -62,6 +62,11 @@ class Game extends Model
         return $this->hasMany(Review::class)->latest();
     }
 
+    public function comments(): HasMany
+    {
+        return $this->hasMany(GameComment::class);
+    }
+
     public function toSearchableArray(): array
     {
         return [