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/Services/Posts/PostShareService.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace App\Services\Posts;
+
+use App\Models\Artwork;
+use App\Models\Post;
+use App\Models\PostTarget;
+use App\Models\User;
+use App\Services\ContentSanitizer;
+use Carbon\Carbon;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Validation\ValidationException;
+
+class PostShareService
+{
+    /**
+     * Share an artwork to a user's profile feed.
+     *
+     * Enforces:
+     *  - artwork must be public
+     *  - no duplicate share within 24 hours (for same user+artwork)
+     *
+     * @throws ValidationException
+     */
+    public function shareArtwork(
+        User $user,
+        Artwork $artwork,
+        ?string $body,
+        string $visibility = Post::VISIBILITY_PUBLIC,
+    ): Post {
+        // Ensure artwork is shareable
+        if (! $artwork->is_public || ! $artwork->is_approved) {
+            throw ValidationException::withMessages([
+                'artwork_id' => ['This artwork cannot be shared because it is not publicly available.'],
+            ]);
+        }
+
+        // Duplicate share prevention: same user + same artwork within 24h
+        $alreadyShared = Post::where('user_id', $user->id)
+            ->where('type', Post::TYPE_ARTWORK_SHARE)
+            ->where('created_at', '>=', Carbon::now()->subHours(24))
+            ->whereHas('targets', function ($q) use ($artwork) {
+                $q->where('target_type', 'artwork')->where('target_id', $artwork->id);
+            })
+            ->exists();
+
+        if ($alreadyShared) {
+            throw ValidationException::withMessages([
+                'artwork_id' => ['You already shared this artwork recently. Please wait 24 hours before sharing it again.'],
+            ]);
+        }
+
+        $sanitizedBody = $body ? ContentSanitizer::render($body) : null;
+
+        return DB::transaction(function () use ($user, $artwork, $sanitizedBody, $visibility) {
+            $post = Post::create([
+                'user_id'    => $user->id,
+                'type'       => Post::TYPE_ARTWORK_SHARE,
+                'visibility' => $visibility,
+                'body'       => $sanitizedBody,
+            ]);
+
+            PostTarget::create([
+                'post_id'     => $post->id,
+                'target_type' => 'artwork',
+                'target_id'   => $artwork->id,
+            ]);
+
+            return $post;
+        });
+    }
+}