Improve LinkedIn UI and scheduling robustness

- When LinkedIn is connected, replace the Connect button with a pencil
  (test post to first personal profile) and a reconnect arrow icon
- Add Process Now button to the Schedule tab that shells out to the cron
  script on demand and shows its output inline
- Add flock() to the cron script to prevent overlapping runs
- Fix cron install path comment to match deployed location

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/process-scheduled-posts.php

@@ -2,13 +2,17 @@
 /**
  * Cron-driven scheduled social post processor.
  *
- * Install once on the host (as the apache user):
- *   * * * * * /usr/bin/php /opt/hackmancms/bin/process-scheduled-posts.php >> /var/log/hackman_scheduled_posts.log 2>&1
+ * Install once on the host (as root, in /etc/cron.d/hackmancms):
+ *   * * * * * www-data /usr/bin/php /var/www/html/hackmancms/bin/process-scheduled-posts.php >> /var/log/hackman_scheduled_posts.log 2>&1
  */
 
 define('ROOT', dirname(__DIR__));
 require ROOT . '/lib/bootstrap.php';
 
+// Prevent overlapping runs
+$_lock_fp = fopen(sys_get_temp_dir() . '/hackman_schedule.lock', 'c');
+if (!$_lock_fp || !flock($_lock_fp, LOCK_EX | LOCK_NB)) exit(0);
+
 $now = gmdate('Y-m-d H:i:s');
 
 // Load all due pending targets grouped by post