Fix plugin routing, missing includes, and make SCRIPTS_DIR configurable

- Rename varaverk.page → Varaverk.page: unRAID nginx only routes URLs
  starting with a capital letter (~^/[A-Z].*), lowercase caused 404
- Add require_once config.php to include/scheduler.php and include/docs.php:
  both used SCRIPTS_DIR constant without including the file that defines it
- Replace hardcoded SCRIPTS_DIR with cfg-file-backed setting: reads from
  /boot/config/plugins/varaverk/varaverk.cfg, falls back to default on first run
- Add Plugin Settings card to Config tab with scripts path field
- Add api/settings.php to write the cfg file (validates directory exists)

Plugin/usr/local/emhttp/plugins/varaverk/include/scheduler.php

@@ -2,6 +2,8 @@
 // Scheduler — manages schedule.json and /etc/cron.d/varaverk.
 // schedule.json is per-host, never synced.
 
+require_once __DIR__ . '/config.php';
+
 define('SCHEDULE_FILE', '/boot/config/plugins/varaverk/schedule.json');
 define('CRON_FILE',     '/etc/cron.d/varaverk');
 define('CRON_USER',     'root');