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>

app/Providers/AppServiceProvider.php

@@ -2,8 +2,10 @@
 
 namespace App\Providers;
 
+use App\Support\Publishing\LinkedInPublisher;
 use App\Support\Publishing\PublisherManager;
 use App\Support\Publishing\RedditPublisher;
+use App\Support\Publishing\TwitterPublisher;
 use App\Support\Publishing\WordPressPublisher;
 use Illuminate\Support\ServiceProvider;
 
@@ -17,6 +19,8 @@ class AppServiceProvider extends ServiceProvider
         $this->app->singleton(PublisherManager::class, fn ($app) => new PublisherManager([
             $app->make(WordPressPublisher::class),
             $app->make(RedditPublisher::class),
+            $app->make(TwitterPublisher::class),
+            $app->make(LinkedInPublisher::class),
         ]));
     }