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/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "twitter-post-sidecar",
+  "private": true,
+  "version": "1.0.0",
+  "description": "Playwright acceptance test that logs into Twitter/X and creates a post, invoked via run.sh",
+  "scripts": {
+    "test": "playwright test"
+  },
+  "devDependencies": {
+    "@playwright/test": "^1.61.1"
+  }
+}