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/components/UpdateBanner/UpdateBanner.tsx

@@ -1,10 +1,12 @@
 import { useState } from 'react'
+import { usePack } from '../../i18n'
 
 // UpdateBanner gently floats down from the top when a newer build has been
 // deployed, inviting a refresh. Mandarin-first copy (north star Note #17), soft
 // rose styling, and a clear primary action. Dismissable — if she dismisses it,
 // the next deploy (or reload) will surface a fresh one.
 export function UpdateBanner() {
+  const t = usePack()
   const [dismissed, setDismissed] = useState(false)
   if (dismissed) return null
 
@@ -30,7 +32,7 @@ export function UpdateBanner() {
             className="truncate text-sm font-bold leading-snug"
             style={{ color: 'var(--color-plum)' }}
           >
-            有新版本啦
+            {t.update.available}
           </p>
           <p className="truncate text-xs leading-snug" style={{ color: 'var(--color-muted)' }}>
             A new version is ready — refresh to update.
@@ -44,13 +46,13 @@ export function UpdateBanner() {
           onMouseEnter={(e) => (e.currentTarget.style.background = 'var(--color-accent-hover)')}
           onMouseLeave={(e) => (e.currentTarget.style.background = 'var(--color-accent)')}
         >
-          刷新 · Refresh
+          {t.update.refresh}
         </button>
         <button
           type="button"
           onClick={() => setDismissed(true)}
-          aria-label="稍后再说 · Dismiss"
-          title="稍后再说 · Dismiss"
+          aria-label={t.update.dismiss}
+          title={t.update.dismiss}
           className="shrink-0 rounded-full px-1.5 text-lg leading-none transition-colors"
           style={{ color: 'var(--color-muted)' }}
           onMouseEnter={(e) => (e.currentTarget.style.color = 'var(--color-plum)')}