Phase 19: the copy stops being hardcoded Mandarin

Every `中文 · English` string moves out of ~29 components into
web/src/i18n: one Pack type, a verbatim zh pack, and two ways to read
it — usePack() for components, pack() for the modules that build a line
when something happens rather than when something renders.

Anything with a value in it is a function on the pack rather than a
template at the call site, English pluralisation included: word order
isn't universal, and a pack author has to be able to move the number.
The roster constants (tones, rewrite styles, export formats, companions)
keep only value + emoji, so a label can't drift from its key.

On the server, internal/llm/lang.go replaces "Simplified Chinese" in the
three prompts that actually name her language. pt-PT is spelled
"European Portuguese (pt-PT, never Brazilian Portuguese)" in the prompt
itself, and each Lang carries her word for "why" so the tutor prompt
still recognises the question when she asks it her way.

pair_lang reaches the model through the row-scoped query each handler
already ran — the one that proves she owns the document — rather than a
second lookup that could disagree with it.

Also records Phase 18's deploy: migration 0011 rehearsed against a copy
of the live VPS database, then applied for real.

web/src/hooks/useSession.ts

@@ -1,6 +1,7 @@
 import { useEffect, useState } from 'react'
 import { api, onUnauthorized, type Me } from '../api/client'
 import { setPrefsScope } from '../lib/prefs'
+import { setPackLang } from '../i18n'
 
 // useSession tracks who is writing, and notices the moment the server stops
 // recognising them.
@@ -25,6 +26,10 @@ export function useSession() {
         // shared — and the first account on this browser inherits whatever was
         // set back when Petal had no accounts at all.
         setPrefsScope(user.id)
+        // …and so does the language Petal speaks back. Until this point the app
+        // renders the default pack; a writer on another pair sees her own copy
+        // from here on, without a reload.
+        setPackLang(user.pair_lang)
         setMe(user)
       })
       .catch(() => {