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/README.md

@@ -0,0 +1,27 @@
+# Twitter/X Post Sidecar
+
+A Playwright acceptance test that logs into Twitter/X with supplied credentials and creates a post.
+
+## One-time setup
+
+    cd sidecars/twitter-post
+    npm install
+    npx playwright install chromium
+
+## Usage
+
+    ./run.sh --username=YOUR_USERNAME --password=YOUR_PASSWORD \
+      --tweet="Your tweet text here" [--headed]
+
+Prints a single JSON line to stdout and exits 0 on success, exits 1 on failure:
+
+    {"success": true, "url": "https://twitter.com/username/status/..."}
+    {"success": false, "error": "Login failed: Invalid credentials"}
+
+## Notes
+
+- On a failed run, Playwright retains a screenshot and trace under `test-results/` for debugging.
+  These are gitignored and not committed, but they persist on disk.
+- Twitter's UI changes frequently. If posting fails, check the selectors in `tests/twitter-post.spec.js`
+  and update them using: `npx playwright codegen https://twitter.com/i/flow/login`
+- Twitter may show CAPTCHAs or 2FA prompts which will cause the automation to fail.