Add Twitter and LinkedIn publishers with Playwright sidecars

- Create twitter-post sidecar with Playwright for browser automation
- Create linkedin-post sidecar with Playwright for browser automation
- Add TwitterPublisher class that calls twitter-post sidecar
- Add LinkedInPublisher class that calls linkedin-post sidecar
- Register both publishers in PublisherManager via AppServiceProvider
- Update Posts index view to show Publish button for twitter/linkedin types

Both publishers use the same pattern as RedditPublisher:
- No API keys required (uses stored credentials from Social Accounts)
- Browser automation via Playwright
- Returns JSON result with success/failure and URL

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

resources/views/livewire/posts/index.blade.php

@@ -47,7 +47,7 @@
                     <td class="p-3">{{ $post->scheduled_at?->format('Y-m-d H:i') ?? '—' }}</td>
                     <td class="p-3">{{ $post->socialAccount?->username ?? '—' }}</td>
                     <td class="p-3 text-right space-x-2">
-                        @if (in_array($post->type, ['blog', 'reddit']) && in_array($post->status, ['draft', 'scheduled', 'failed']))
+                        @if (in_array($post->type, ['blog', 'reddit', 'twitter', 'linkedin']) && in_array($post->status, ['draft', 'scheduled', 'failed']))
                             <button wire:click="publish({{ $post->id }})" class="text-green-700">Publish</button>
                         @endif
                         <button wire:click="edit({{ $post->id }})" class="text-blue-600">Edit</button>