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
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