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/Auth/SignInOverlay.tsx

@@ -6,6 +6,8 @@
 // again as an errand, not an error. The editor stays visible behind the scrim
 // (dimmed, still there) for the same reason: nothing has been taken away.
 
+import { usePack } from '../../i18n'
+
 interface Props {
   // Whether there is unsaved writing waiting on this device, which changes the
   // reassurance from a promise to a statement of fact.
@@ -13,6 +15,7 @@ interface Props {
 }
 
 export function SignInOverlay({ hasDraft }: Props) {
+  const t = usePack()
   return (
     <div
       role="dialog"
@@ -39,21 +42,17 @@ export function SignInOverlay({ hasDraft }: Props) {
           className="mt-3 text-lg font-bold"
           style={{ color: 'var(--color-plum)' }}
         >
-          请重新登录
+          {t.auth.title}
         </h2>
         <p className="text-sm font-semibold" style={{ color: 'var(--color-muted)' }}>
-          Please sign in again
+          {t.auth.titleEn}
         </p>
 
         <p className="mt-4 text-sm leading-relaxed" style={{ color: 'var(--color-plum)' }}>
-          {hasDraft
-            ? '你刚写的内容已经安全地留在这台电脑上，登录后会自动接着保存。'
-            : '登录状态过期了。你的文字都已经保存好了。'}
+          {hasDraft ? t.auth.bodyWithDraft : t.auth.bodyPlain}
         </p>
         <p className="mt-1 text-xs leading-relaxed" style={{ color: 'var(--color-muted)' }}>
-          {hasDraft
-            ? "What you just wrote is safe on this device — it'll save itself once you're back in."
-            : 'Your session expired. Everything you wrote is already saved.'}
+          {hasDraft ? t.auth.bodyWithDraftEn : t.auth.bodyPlainEn}
         </p>
 
         <a
@@ -63,7 +62,7 @@ export function SignInOverlay({ hasDraft }: Props) {
           onMouseEnter={(e) => (e.currentTarget.style.background = 'var(--color-accent-hover)')}
           onMouseLeave={(e) => (e.currentTarget.style.background = 'var(--color-accent)')}
         >
-          去登录 · Sign in
+          {t.auth.signIn}
         </a>
       </div>
     </div>