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/docs.php

@@ -2,6 +2,8 @@
 // Docs — markdown file discovery, $VAR substitution, and rendering.
 // Requires parsedown or similar. Falls back to <pre> if not available.
 
+require_once __DIR__ . '/config.php';
+
 define('PARSEDOWN_PATH', '/usr/local/emhttp/plugins/varaverk/lib/Parsedown.php');
 
 function vv_docs_tree(): array {