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>

resources/views/users/show.blade.php

@@ -58,7 +58,7 @@
                 </div>
             </div>
 
-            <div class="flex gap-2">
+            <div class="flex gap-2 flex-wrap">
                 @auth
                 @if(auth()->id() !== $user->id)
                 <div x-data="{
@@ -92,6 +92,14 @@
                    class="px-4 py-2 border border-white/10 hover:border-white/20 text-slate-300 hover:text-white text-sm font-medium rounded-lg transition-colors">
                     View Library
                 </a>
+                @auth
+                @if(auth()->id() !== $user->id)
+                <a href="{{ route('users.compare', $user->username) }}"
+                   class="px-4 py-2 border border-purple-500/40 hover:border-purple-500 text-purple-300 hover:text-white text-sm font-medium rounded-lg transition-colors">
+                    Compare Libraries
+                </a>
+                @endif
+                @endauth
             </div>
         </div>
     </div>