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>

tests/Feature/Livewire/PostsTest.php

@@ -0,0 +1,81 @@
+<?php
+
+use App\Livewire\Posts\Index;
+use App\Models\Post;
+use App\Models\Site;
+use App\Models\SocialAccount;
+use Livewire\Livewire;
+
+test('posts index page lists existing posts', function () {
+    $site = Site::factory()->create(['name' => 'My Blog']);
+    Post::factory()->create(['site_id' => $site->id, 'title' => 'Hello World Post']);
+
+    $this->get(route('posts.index'))
+        ->assertOk()
+        ->assertSee('Hello World Post')
+        ->assertSee('My Blog');
+});
+
+test('a new post can be created with required fields only', function () {
+    $site = Site::factory()->create();
+
+    Livewire::test(Index::class)
+        ->set('siteId', $site->id)
+        ->set('type', 'blog')
+        ->set('body', 'New post body')
+        ->call('save');
+
+    $post = Post::where('body', 'New post body')->first();
+    expect($post)->not->toBeNull();
+    expect($post->site_id)->toBe($site->id);
+    expect($post->status)->toBe('draft');
+});
+
+test('a new post can be created with an account, title, status, and schedule', function () {
+    $site = Site::factory()->create();
+    $account = SocialAccount::factory()->create();
+
+    Livewire::test(Index::class)
+        ->set('siteId', $site->id)
+        ->set('socialAccountId', $account->id)
+        ->set('type', 'twitter')
+        ->set('title', 'Launch')
+        ->set('body', 'We launched')
+        ->set('status', 'scheduled')
+        ->set('scheduledAt', '2026-08-01T09:00')
+        ->call('save');
+
+    $post = Post::where('title', 'Launch')->first();
+    expect($post->social_account_id)->toBe($account->id);
+    expect($post->status)->toBe('scheduled');
+    expect($post->scheduled_at)->not->toBeNull();
+});
+
+test('site, type, and body are required to create a post', function () {
+    Livewire::test(Index::class)
+        ->set('siteId', '')
+        ->set('type', '')
+        ->set('body', '')
+        ->call('save')
+        ->assertHasErrors(['siteId', 'type', 'body']);
+});
+
+test('an existing post can be edited', function () {
+    $site = Site::factory()->create();
+    $post = Post::factory()->create(['site_id' => $site->id, 'title' => 'Old Title']);
+
+    Livewire::test(Index::class)
+        ->call('edit', $post->id)
+        ->set('title', 'New Title')
+        ->call('save');
+
+    expect($post->fresh()->title)->toBe('New Title');
+});
+
+test('a post can be deleted', function () {
+    $post = Post::factory()->create();
+
+    Livewire::test(Index::class)->call('delete', $post->id);
+
+    expect(Post::find($post->id))->toBeNull();
+});