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/App.tsx

@@ -18,6 +18,7 @@ import { useSession } from './hooks/useSession'
 import { takeDraft } from './lib/drafts'
 import { useVersionWatch } from './hooks/useVersionWatch'
 import { PetalFall } from './effects/PetalFall'
+import { usePack } from './i18n'
 import { useNightMode } from './hooks/useNightMode'
 import { playSuggestionSound } from './audio/sounds'
 
@@ -30,6 +31,7 @@ export default function App() {
   // Who's writing, and whether the server still recognises them. `signedOut`
   // flips the moment any call comes back 401.
   const { me, signedOut } = useSession()
+  const t = usePack()
   // A real account to sign out of, as opposed to the hardcoded local user a
   // build without auth configured runs as.
   const account = me && me.id !== 'local' ? { name: me.display_name || me.email } : null
@@ -239,7 +241,8 @@ export default function App() {
     setDocText('')
   }, [saveNow, isBlankDraft])
 
-  // Duplicate a document: copy its body/tone into a fresh doc titled "… (副本)",
+  // Duplicate a document: copy its body/tone into a fresh doc under the pack's
+  // "copy" title,
   // then open the copy. Tags aren't carried over (a fresh start for the copy).
   const handleDuplicate = useCallback(
     async (id: string) => {
@@ -247,7 +250,7 @@ export default function App() {
         await saveNow() // flush in case we're duplicating the open doc
         const src = await api.getDoc(id)
         const fresh = await api.createDoc()
-        const dupTitle = `${src.title?.trim() || 'Untitled'} (副本)`
+        const dupTitle = t.app.duplicateTitle(src.title?.trim() || 'Untitled')
         const updated = await api.updateDoc(fresh.id, {
           title: dupTitle,
           content: src.content,
@@ -451,7 +454,7 @@ export default function App() {
           }}
         >
           <span aria-hidden>🌷</span>
-          <span>词汇花园</span>
+          <span>{t.app.garden}</span>
           <span style={{ color: 'var(--color-muted)' }}>· Garden</span>
         </button>
       </header>
@@ -514,7 +517,7 @@ export default function App() {
                       }}
                     >
                       <span aria-hidden>🕘</span>
-                      <span>历史</span>
+                      <span>{t.app.history}</span>
                       <span style={{ color: 'var(--color-muted)' }}>· History</span>
                     </button>
                     <div className="petal-no-print">