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/Policies/GameCommentPolicy.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Policies;
+
+use App\Models\GameComment;
+use App\Models\User;
+
+class GameCommentPolicy
+{
+    public function delete(User $user, GameComment $comment): bool
+    {
+        return $user->id === $comment->user_id;
+    }
+}