Lang settings tab: flag-grid language picker, bootstrap es+zh targets

- New editor-shell tab (lang): translatable languages as clickable flag
  tiles (shared langs.js mapping of the 28 Seed-X languages to flags),
  saved via the settings round-trip; translator service URL(s) shown.
- Bootstrap seeds translate_langs with Spanish and Chinese (the original
  language is never a target).
- GET /_api/settings also returns primary_lang.
- PageEditor's flag/name helpers move to the shared langs.js.

docs/localization.md

@@ -282,8 +282,10 @@ Frames are JSON-encoded tagged msgspec structs (`pagerite/translate.py`;
   translation of the connection's current job, matching it by (lang, key).
 
 Which languages get translated is **server-configured**:
-`Data.translate_langs` (presence-key dict, read/set via `/_api/settings`
-as `translate_langs`; no editing UI yet). The dispatcher offers a
+`Data.translate_langs` (presence-key dict, bootstrapped to Spanish and
+Chinese — the original language is never a target — edited in the editor
+shell's localization tab or set via `/_api/settings` as `translate_langs`).
+The dispatcher offers a
 connection jobs only in `wanted ∩ capable`; a connection without overlap
 simply stays idle.