fix: apply locale to DOM immediately on save — no reload needed

Add applyLocaleToDOM() which walks [data-i18n] elements and re-stamps
their textContent from t(). Called after setLocale() in saveSettings()
so the settings panel labels, checkboxes, and save button update live.
Also called on boot after /api/settings resolves so Chinese persists
without flicker on reload.

- static/i18n.js: add applyLocaleToDOM() function
- static/index.html: add data-i18n attributes to all settings panel
  static text nodes (labels, checkbox spans, save button)
- static/panels.js: call applyLocaleToDOM() + syncTopbar() after save
- static/boot.js: call applyLocaleToDOM() alongside setLocale() on boot

static/panels.js

@@ -1098,9 +1098,11 @@ async function saveSettings(andClose){
     window._botName=body.bot_name;
     if(typeof applyBotName==='function') applyBotName();
     if(typeof setLocale==='function') setLocale(language);
+    if(typeof applyLocaleToDOM==='function') applyLocaleToDOM();
     _settingsDirty=false; _settingsThemeOnOpen=theme;
     const bar=$('settingsUnsavedBar'); if(bar) bar.style.display='none';
     renderMessages();
+    if(typeof syncTopbar==='function') syncTopbar();
     if(typeof renderSessionList==='function') renderSessionList();
     showToast(t('settings_saved'));
     $('settingsOverlay').style.display='none';