Add configuration backup/restore with settings modal

Backend: GET /api/v1/system/backup bundles all 11 store JSON files into a
single downloadable backup with metadata envelope. POST /api/v1/system/restore
validates and writes stores atomically, then schedules a delayed server restart
via detached restart.ps1 subprocess.

Frontend: Settings modal (gear button in header) with Download Backup and
Restore from Backup buttons. Restore shows confirm dialog, uploads via
multipart FormData, then displays fullscreen restart overlay that polls
/health until the server comes back and reloads the page.

Locales: en, ru, zh translations for all settings.* keys.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/features/settings.js

@@ -0,0 +1,137 @@
+/**
+ * Settings — backup / restore configuration.
+ */
+
+import { apiKey } from '../core/state.js';
+import { API_BASE, fetchWithAuth } from '../core/api.js';
+import { Modal } from '../core/modal.js';
+import { showToast, showConfirm } from '../core/ui.js';
+import { t } from '../core/i18n.js';
+
+// Simple modal (no form / no dirty check needed)
+const settingsModal = new Modal('settings-modal');
+
+export function openSettingsModal() {
+    document.getElementById('settings-error').style.display = 'none';
+    settingsModal.open();
+}
+
+export function closeSettingsModal() {
+    settingsModal.forceClose();
+}
+
+// ─── Backup ────────────────────────────────────────────────
+
+export async function downloadBackup() {
+    try {
+        const resp = await fetchWithAuth('/system/backup', { timeout: 30000 });
+        if (!resp.ok) {
+            const err = await resp.json().catch(() => ({}));
+            throw new Error(err.detail || `HTTP ${resp.status}`);
+        }
+        const blob = await resp.blob();
+        const disposition = resp.headers.get('Content-Disposition') || '';
+        const match = disposition.match(/filename="(.+?)"/);
+        const filename = match ? match[1] : 'ledgrab-backup.json';
+
+        const a = document.createElement('a');
+        a.href = URL.createObjectURL(blob);
+        a.download = filename;
+        document.body.appendChild(a);
+        a.click();
+        a.remove();
+        URL.revokeObjectURL(a.href);
+
+        showToast(t('settings.backup.success'), 'success');
+    } catch (err) {
+        console.error('Backup download failed:', err);
+        showToast(t('settings.backup.error') + ': ' + err.message, 'error');
+    }
+}
+
+// ─── Restore ───────────────────────────────────────────────
+
+export async function handleRestoreFileSelected(input) {
+    const file = input.files[0];
+    input.value = '';
+    if (!file) return;
+
+    const confirmed = await showConfirm(t('settings.restore.confirm'));
+    if (!confirmed) return;
+
+    try {
+        const formData = new FormData();
+        formData.append('file', file);
+
+        const resp = await fetch(`${API_BASE}/system/restore`, {
+            method: 'POST',
+            headers: { 'Authorization': `Bearer ${apiKey}` },
+            body: formData,
+        });
+
+        if (!resp.ok) {
+            const err = await resp.json().catch(() => ({}));
+            throw new Error(err.detail || `HTTP ${resp.status}`);
+        }
+
+        const data = await resp.json();
+        showToast(data.message || t('settings.restore.success'), 'success');
+        settingsModal.forceClose();
+
+        if (data.restart_scheduled) {
+            showRestartOverlay();
+        }
+    } catch (err) {
+        console.error('Restore failed:', err);
+        showToast(t('settings.restore.error') + ': ' + err.message, 'error');
+    }
+}
+
+// ─── Restart overlay ───────────────────────────────────────
+
+function showRestartOverlay() {
+    const overlay = document.createElement('div');
+    overlay.id = 'restart-overlay';
+    overlay.style.cssText =
+        'position:fixed;inset:0;z-index:100000;display:flex;flex-direction:column;' +
+        'align-items:center;justify-content:center;background:rgba(0,0,0,0.85);color:#fff;font-size:1.2rem;';
+    overlay.innerHTML =
+        '<div class="spinner" style="width:48px;height:48px;border:4px solid rgba(255,255,255,0.3);' +
+        'border-top-color:#fff;border-radius:50%;animation:spin 0.8s linear infinite;margin-bottom:1rem;"></div>' +
+        `<div id="restart-msg">${t('settings.restore.restarting')}</div>`;
+
+    // Add spinner animation if not present
+    if (!document.getElementById('restart-spinner-style')) {
+        const style = document.createElement('style');
+        style.id = 'restart-spinner-style';
+        style.textContent = '@keyframes spin{to{transform:rotate(360deg)}}';
+        document.head.appendChild(style);
+    }
+
+    document.body.appendChild(overlay);
+    pollHealth();
+}
+
+function pollHealth() {
+    const start = Date.now();
+    const maxWait = 30000;
+    const interval = 1500;
+
+    const check = async () => {
+        if (Date.now() - start > maxWait) {
+            const msg = document.getElementById('restart-msg');
+            if (msg) msg.textContent = t('settings.restore.restart_timeout');
+            return;
+        }
+        try {
+            const resp = await fetch('/health', { signal: AbortSignal.timeout(3000) });
+            if (resp.ok) {
+                window.location.reload();
+                return;
+            }
+        } catch { /* server still down */ }
+        setTimeout(check, interval);
+    };
+    // Wait a moment before first check to let the server shut down
+    setTimeout(check, 2000);
+}