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>

sidecars/twitter-post/playwright.config.js

@@ -0,0 +1,14 @@
+/** @type {import('@playwright/test').PlaywrightTestConfig} */
+const config = {
+  testDir: './tests',
+  fullyParallel: false,
+  workers: 1,
+  reporter: 'json',
+  use: {
+    trace: 'retain-on-failure',
+    screenshot: 'only-on-failure',
+    video: 'retain-on-failure',
+  },
+};
+
+export default config;