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>

database/migrations/2026_05_30_000001_create_game_comments_table.php

@@ -0,0 +1,25 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('game_comments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('user_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('game_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('parent_id')->nullable()->constrained('game_comments')->cascadeOnDelete();
+            $table->text('body');
+            $table->timestamps();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('game_comments');
+    }
+};