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/DocList/DocListItem.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import type { DocSummary, Tag, TagColor } from '../../api/client'
 import { TagChip } from './TagChip'
 import { TagPicker } from './TagPicker'
+import { usePack } from '../../i18n'
 
 interface Props {
   doc: DocSummary
@@ -27,6 +28,7 @@ export function DocListItem({
   onCreateTag,
 }: Props) {
   const [picking, setPicking] = useState(false)
+  const t = usePack()
   const assignedIds = new Set(doc.tags.map((t) => t.id))
 
   return (
@@ -70,7 +72,7 @@ export function DocListItem({
         <button
           type="button"
           aria-label="Duplicate document"
-          title="副本 · Duplicate"
+          title={t.docs.duplicate}
           onClick={(e) => {
             e.stopPropagation()
             onDuplicate()