feat: optional auth + backup/restore reliability fixes

Auth is now optional: when `auth.api_keys` is empty, all endpoints are
open (no login screen, no Bearer tokens). Health endpoint reports
`auth_required` so the frontend knows which mode to use.

Backup/restore fixes:
- Auto-backup uses atomic writes (was `write_text`, risked corruption)
- Startup backup skipped if recent backup exists (<5 min cooldown),
  preventing rapid restarts from rotating out good backups
- Restore rejects all-empty backups to prevent accidental data wipes
- Store saves frozen after restore to prevent stale in-memory data
  from overwriting freshly-restored files before restart completes
- Missing stores during restore logged as warnings
- STORE_MAP completeness verified at startup against StorageConfig

server/src/wled_controller/static/js/global.d.ts

@@ -18,6 +18,7 @@ interface Window {
 
     // ─── Core / state ───
     setApiKey: (key: string | null) => void;
+    _authRequired: boolean | undefined;
 
     // ─── Visual effects (called from inline <script>) ───
     _updateBgAnimAccent: (accent: string) => void;
@@ -372,6 +373,11 @@ interface Window {
     saveExternalUrl: (...args: any[]) => any;
     getBaseOrigin: (...args: any[]) => any;
 
+    // ─── Appearance ───
+    applyStylePreset: (id: string) => void;
+    applyBgEffect: (id: string) => void;
+    renderAppearanceTab: () => void;
+
     // ─── Overlay spinner internals ───
     overlaySpinnerTimer: ReturnType<typeof setInterval> | null;
     _overlayEscHandler: ((e: KeyboardEvent) => void) | null;