Platform adapter: rename System_Essentials, add Plugin/unraid/adapter.sh, wire call sites

- Rename unRAID_Essentials/ → System_Essentials/ (git detects as rename)
- Add Plugin/unraid/adapter.sh: 13 platform_*() functions providing OS-agnostic API
  for storage health, service management, mover, user scripts, notifications,
  disk temps, and platform command validation
- Update load_config.sh: detect PLATFORM (unraid/truenas/unknown), export SCRIPTS_DIR,
  auto-source Plugin/$PLATFORM/adapter.sh after common.sh
- Wire all call sites: replace direct rc.d, pgrep/pkill, var.ini, dynamix.cfg,
  disks.ini, and validate_unraid_cmd calls with platform_*() functions across
  watchdogs, orchestrators, and System_Essentials scripts
- Update all documentation: rename refs, update webgui escalation logic,
  add platform adapter section to Plugin README, update main README with
  portability vision and corrected self-healing stack description

Plugin/unraid/api/storage.php

@@ -141,7 +141,7 @@ if ($action === 'api_status') {
 
 // ── Setup/renew API keys (local + all partners via SSH) ───────────────────────
 if ($action === 'setup_apikeys' && $_SERVER['REQUEST_METHOD'] === 'POST') {
-    $script = SCRIPTS_DIR . '/unRAID_Essentials/unraid_api_key_renew.sh';
+    $script = SCRIPTS_DIR . '/System_Essentials/unraid_api_key_renew.sh';
     if (!file_exists($script)) {
         echo json_encode(['ok' => false, 'error' => 'unraid_api_key_renew.sh not found']); exit;
     }