web: move the language and theme selectors to the System section

The header carried the save button, the language selector and the theme
selector on every section, although the latter two are set once. They
now live in a Display card in the System section; the header keeps the
title, the unsaved-changes badge and Save to flash.

doc/support-multi-language.md

@@ -11,7 +11,7 @@ All translation logic lives at the top of `html/app.js`:
 - A `LANG` object with one sub-object per language (`en`, `ja`, `zh`)
 - Language auto-detection (browser language, overridden by the
   `rtl_lang` key in `localStorage`, which the language selector in the
-  page header writes)
+  System section writes)
 - `t(key, vars)` looks up a translated string; `{name}` placeholders in
   the string are replaced from `vars`, e.g. `t("v_del_q", {n: vid})`
 - `i18nApply()` applies the dictionary to the static markup once at load