feat: add Posts Livewire CRUD page

Adds the Posts index page (list/create/edit/delete) following the
existing Sites/SocialAccounts pattern, and extends the shared nav
layout with a Posts link.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

routes/web.php

@@ -1,5 +1,6 @@
 <?php
 
+use App\Livewire\Posts\Index as PostsIndex;
 use App\Livewire\Sites\Index as SitesIndex;
 use App\Livewire\SocialAccounts\Index as SocialAccountsIndex;
 use Illuminate\Support\Facades\Route;
@@ -10,3 +11,4 @@ Route::get('/', function () {
 
 Route::get('/sites', SitesIndex::class)->name('sites.index');
 Route::get('/social-accounts', SocialAccountsIndex::class)->name('social-accounts.index');
+Route::get('/posts', PostsIndex::class)->name('posts.index');