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/templates/index.html

@@ -341,14 +341,27 @@
         const savedAccent = localStorage.getItem('accentColor');
         if (savedAccent) applyAccentColor(savedAccent, true);
 
+        // Early-apply saved background effect class (before bundle loads)
+        const savedBgEffect = localStorage.getItem('bgEffect');
+        if (savedBgEffect && savedBgEffect !== 'none') {
+            const effectClasses = { grid: 'bg-effect-grid', mesh: 'bg-effect-mesh', scanlines: 'bg-effect-scanlines', particles: 'bg-effect-particles' };
+            if (effectClasses[savedBgEffect]) document.documentElement.classList.add(effectClasses[savedBgEffect]);
+        }
+
         // Initialize auth state
         function updateAuthUI() {
             const apiKey = localStorage.getItem('wled_api_key');
             const loginBtn = document.getElementById('login-btn');
             const logoutBtn = document.getElementById('logout-btn');
             const tabBar = document.querySelector('.tab-bar');
+            const authDisabled = window._authRequired === false;
 
-            if (apiKey) {
+            if (authDisabled) {
+                // Auth disabled — hide login/logout, always show tabs
+                loginBtn.style.display = 'none';
+                logoutBtn.style.display = 'none';
+                if (tabBar) tabBar.style.display = '';
+            } else if (apiKey) {
                 loginBtn.style.display = 'none';
                 logoutBtn.style.display = 'inline-block';
                 if (tabBar) tabBar.style.display = '';