Let her choose her own pair

Raised by the user, not by the plan: there was no way to change language
in the mobile UI. There was no way anywhere. `users.pair_lang` has been
readable since Phase 19 and writable by nobody — /api/me was GET-only and
Upsert deliberately skips the column — which is also why "no pt-PT account
exists yet" has stood through two phases. Nothing could create one.

PATCH /api/me answers with the whole user rather than 204, so the client
re-reads the pair from the server instead of trusting its own request. One
write reaches everything: langpack, Hunspell dictionary, Piper voice,
lexicon provider and prompt language all read the column at use time.

The server refuses a pair it has no copy for, and auth.shippedPairs is
deliberately not internal/llm's list. That one names pairs the prompts can
talk about (fr and es, since Phase 19); this one names pairs Petal can
render itself in, which needs a langpack. Storing fr today would strand
her on Chinese with no way back except a lucky guess at a button she
cannot read.

The picker sits in the sidebar footer because the sidebar is the mobile
drawer — always one tap away. The status bar exists only while a document
is open, which is the wrong moment to find the app speaking a language you
can't read. Each language names itself, 中文 and Português: the one place
bilingual copy would get in the way.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

web/src/api/client.ts

@@ -251,6 +251,13 @@ export const api = {
   // the hardcoded local user, so the frontend needs no separate mode for it.
   me: () => req<Me>('/me'),
 
+  // Move to another (English + X) pair. Answers with the whole updated user, so
+  // the caller re-reads the pair from the server rather than assuming its own
+  // request took — a code the server won't ship comes back 400 and the app is
+  // still on a language it can render.
+  setPairLang: (lang: string) =>
+    req<Me>('/me', { method: 'PATCH', body: JSON.stringify({ pair_lang: lang }) }),
+
   listDocs: () => req<DocSummary[]>('/docs'),
   createDoc: () => req<Document>('/docs', { method: 'POST' }),
   getDoc: (id: string) => req<Document>(`/docs/${id}`),