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/Http/Requests/Posts/ShareArtworkRequest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Http\Requests\Posts;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ShareArtworkRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return (bool) $this->user();
+    }
+
+    public function rules(): array
+    {
+        return [
+            'body'       => ['nullable', 'string', 'max:2000'],
+            'visibility' => ['required', 'string', 'in:public,followers,private'],
+        ];
+    }
+}