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/Http/Controllers/GameCommentController.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Models\Game;
+use App\Models\GameComment;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+
+class GameCommentController extends Controller
+{
+    public function store(Request $request, Game $game): JsonResponse
+    {
+        $data = $request->validate([
+            'body'      => 'required|string|max:1000',
+            'parent_id' => 'nullable|integer|exists:game_comments,id',
+        ]);
+
+        if (isset($data['parent_id'])) {
+            $parent = GameComment::findOrFail($data['parent_id']);
+            abort_if($parent->game_id !== $game->id, 422, 'Parent comment does not belong to this game.');
+        }
+
+        $comment = $game->comments()->create([
+            'user_id'   => auth()->id(),
+            'body'      => $data['body'],
+            'parent_id' => $data['parent_id'] ?? null,
+        ]);
+
+        $comment->load('user');
+
+        return response()->json([
+            'id'           => $comment->id,
+            'body'         => $comment->body,
+            'user'         => ['name' => $comment->user->name, 'username' => $comment->user->username],
+            'user_id'      => $comment->user_id,
+            'created_at'   => $comment->created_at->diffForHumans(),
+            'replies_count' => 0,
+        ], 201);
+    }
+
+    public function destroy(GameComment $comment): JsonResponse
+    {
+        $this->authorize('delete', $comment);
+        $comment->delete();
+
+        return response()->json(['deleted' => true]);
+    }
+}