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/run.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$SCRIPT_DIR"
+
+USERNAME=""
+PASSWORD=""
+TWEET=""
+HEADLESS_FLAG="true"
+
+for arg in "$@"; do
+  case "$arg" in
+    --username=*) USERNAME="${arg#*=}" ;;
+    --password=*) PASSWORD="${arg#*=}" ;;
+    --tweet=*) TWEET="${arg#*=}" ;;
+    --headed) HEADLESS_FLAG="false" ;;
+    *)
+      UNKNOWN_FLAG="${arg%%=*}"
+      echo "{\"success\": false, \"error\": \"Unknown argument: $UNKNOWN_FLAG\"}"
+      exit 1
+      ;;
+  esac
+done
+
+MISSING=""
+[ -z "$USERNAME" ] && MISSING="$MISSING --username"
+[ -z "$PASSWORD" ] && MISSING="$MISSING --password"
+[ -z "$TWEET" ] && MISSING="$MISSING --tweet"
+
+if [ -n "$MISSING" ]; then
+  echo "{\"success\": false, \"error\": \"Missing required argument(s):$MISSING\"}"
+  exit 1
+fi
+
+export TWITTER_USERNAME="$USERNAME"
+export TWITTER_PASSWORD="$PASSWORD"
+export TWEET_TEXT="$TWEET"
+export HEADLESS="$HEADLESS_FLAG"
+
+set +e
+npx playwright test tests/twitter-post.spec.js
+PLAYWRIGHT_EXIT_CODE=$?
+set -e
+
+RESULT_FILE="test-results/result.json"
+
+if [ -f "$RESULT_FILE" ]; then
+  cat "$RESULT_FILE"
+  echo
+else
+  echo "{\"success\": false, \"error\": \"Test did not produce a result file\"}"
+  exit 1
+fi
+
+exit "$PLAYWRIGHT_EXIT_CODE"