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/Controllers/Api/Posts/PostShareController.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace App\Http\Controllers\Api\Posts;
+
+use App\Events\Posts\ArtworkShared;
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Posts\ShareArtworkRequest;
+use App\Models\Artwork;
+use App\Services\Posts\PostFeedService;
+use App\Services\Posts\PostShareService;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Support\Facades\RateLimiter;
+
+class PostShareController extends Controller
+{
+    public function __construct(
+        private PostShareService $shareService,
+        private PostFeedService  $feedService,
+    ) {}
+
+    /**
+     * POST /api/posts/share/artwork/{artwork_id}
+     * payload: { body?, visibility }
+     */
+    public function shareArtwork(ShareArtworkRequest $request, int $artworkId): JsonResponse
+    {
+        $user    = $request->user();
+        $artwork = Artwork::findOrFail($artworkId);
+
+        // Rate limit: 10 artwork shares per hour
+        $key = 'share_artwork:' . $user->id;
+        if (RateLimiter::tooManyAttempts($key, 10)) {
+            $seconds = RateLimiter::availableIn($key);
+            return response()->json([
+                'message' => "You're sharing too quickly. Please wait {$seconds} seconds.",
+            ], 429);
+        }
+        RateLimiter::hit($key, 3600);
+
+        $post = $this->shareService->shareArtwork(
+            user:       $user,
+            artwork:    $artwork,
+            body:       $request->input('body'),
+            visibility: $request->input('visibility', 'public'),
+        );
+
+        $post->load(['user', 'user.profile', 'targets', 'targets.artwork', 'targets.artwork.user', 'targets.artwork.user.profile', 'reactions']);
+
+        // Notify original artwork owner (unless self-share)
+        if ($artwork->user_id !== $user->id) {
+            event(new ArtworkShared($post, $artwork, $user));
+        }
+
+        return response()->json([
+            'post' => $this->feedService->formatPost($post, $user->id),
+        ], 201);
+    }
+}