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/PostPinController.php

@@ -0,0 +1,67 @@
+<?php
+
+namespace App\Http\Controllers\Api\Posts;
+
+use App\Http\Controllers\Controller;
+use App\Models\Post;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Gate;
+
+/**
+ * POST   /api/posts/{id}/pin
+ * DELETE /api/posts/{id}/pin
+ */
+class PostPinController extends Controller
+{
+    private const MAX_PINNED = 3;
+
+    public function pin(Request $request, int $id): JsonResponse
+    {
+        $post = Post::where('status', Post::STATUS_PUBLISHED)->findOrFail($id);
+        Gate::authorize('update', $post);
+
+        $user = $request->user();
+
+        // Count existing pinned posts
+        $pinnedCount = Post::where('user_id', $user->id)
+            ->where('is_pinned', true)
+            ->count();
+
+        if ($post->is_pinned) {
+            return response()->json(['message' => 'Post is already pinned.'], 409);
+        }
+
+        if ($pinnedCount >= self::MAX_PINNED) {
+            return response()->json([
+                'message' => 'You can pin a maximum of ' . self::MAX_PINNED . ' posts.',
+            ], 422);
+        }
+
+        $nextOrder = Post::where('user_id', $user->id)
+            ->where('is_pinned', true)
+            ->max('pinned_order') ?? 0;
+
+        $post->update([
+            'is_pinned'    => true,
+            'pinned_order' => $nextOrder + 1,
+        ]);
+
+        return response()->json(['message' => 'Post pinned.', 'post_id' => $post->id]);
+    }
+
+    public function unpin(int $id): JsonResponse
+    {
+        $post = Post::findOrFail($id);
+        Gate::authorize('update', $post);
+
+        if (! $post->is_pinned) {
+            return response()->json(['message' => 'Post is not pinned.'], 409);
+        }
+
+        $post->update(['is_pinned' => false, 'pinned_order' => null]);
+
+        return response()->json(['message' => 'Post unpinned.', 'post_id' => $post->id]);
+    }
+}