feat: forum rich-text editor, emoji picker, mentions, discover nav, feed, uploads, profile

Forum:
- TipTap WYSIWYG editor with full toolbar
- @emoji-mart/react emoji picker (consistent with tweets)
- @mention autocomplete with user search API
- Fix PHP 8.4 parse errors in Blade templates
- Fix thread data display (paginator items)
- Align forum page widths to max-w-5xl

Discover:
- Extract shared _nav.blade.php partial
- Add missing nav links to for-you page
- Add Following link for authenticated users

Feed/Posts:
- Post model, controllers, policies, migrations
- Feed page components (PostComposer, FeedCard, etc)
- Post reactions, comments, saves, reports, sharing
- Scheduled publishing support
- Link preview controller

Profile:
- Profile page components (ProfileHero, ProfileTabs)
- Profile API controller

Uploads:
- Upload wizard enhancements
- Scheduled publish picker
- Studio status bar and readiness checklist

app/Notifications/ArtworkSharedNotification.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Notifications;
+
+use App\Models\Artwork;
+use App\Models\Post;
+use App\Models\User;
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Notifications\Notification;
+
+class ArtworkSharedNotification extends Notification implements ShouldQueue
+{
+    use Queueable;
+
+    public function __construct(
+        public readonly Post    $post,
+        public readonly Artwork $artwork,
+        public readonly User    $sharer,
+    ) {}
+
+    public function via(object $notifiable): array
+    {
+        return ['database'];
+    }
+
+    public function toDatabase(object $notifiable): array
+    {
+        return [
+            'type'         => 'artwork_shared',
+            'post_id'      => $this->post->id,
+            'artwork_id'   => $this->artwork->id,
+            'artwork_title' => $this->artwork->title,
+            'sharer_id'    => $this->sharer->id,
+            'sharer_name'  => $this->sharer->name,
+            'sharer_username' => $this->sharer->username,
+            'message'      => "{$this->sharer->name} shared your artwork "{$this->artwork->title}"",
+            'url'          => "/@{$this->sharer->username}?tab=posts",
+        ];
+    }
+}