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/templates/index.html

@@ -34,6 +34,9 @@
                 <button class="theme-toggle" onclick="toggleTheme()" data-i18n-title="theme.toggle" title="Toggle theme">
                     <span id="theme-icon">🌙</span>
                 </button>
+                <button class="search-toggle" onclick="openSettingsModal()" data-i18n-title="settings.title" title="Settings">
+                    &#x2699;&#xFE0F;
+                </button>
                 <select id="locale-select" onchange="changeLocale()" data-i18n-title="locale.change" title="Change language" style="padding: 4px 8px; border: 1px solid var(--border-color); border-radius: 4px; background: var(--bg-color); color: var(--text-color); font-size: 0.8rem; cursor: pointer;">
                     <option value="en">English</option>
                     <option value="ru">Русский</option>
@@ -128,6 +131,7 @@
     {% include 'modals/test-audio-template.html' %}
     {% include 'modals/value-source-editor.html' %}
     {% include 'modals/test-value-source.html' %}
+    {% include 'modals/settings.html' %}
 
     {% include 'partials/tutorial-overlay.html' %}
     {% include 'partials/image-lightbox.html' %}