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/GameComment.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class GameComment extends Model
+{
+    protected $fillable = ['user_id', 'game_id', 'parent_id', 'body'];
+
+    public function user(): BelongsTo { return $this->belongsTo(User::class); }
+    public function game(): BelongsTo { return $this->belongsTo(Game::class); }
+    public function replies(): HasMany { return $this->hasMany(GameComment::class, 'parent_id')->with('user')->latest(); }
+
+    public function scopeTopLevel(Builder $query): Builder { return $query->whereNull('parent_id'); }
+}