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/Editor/AskPetal.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState } from 'react'
 import { api, streamSuggestionChat, type ChatMessage } from '../../api/client'
+import { usePack } from '../../i18n'
 
 interface Props {
   suggestionId: string
@@ -19,6 +20,7 @@ const CHAT_FONT = "'Nunito', 'PingFang SC', 'Microsoft YaHei', 'Noto Sans CJK SC
 // the card (unmounting) clears it. Each send streams Petal's reply token-by-
 // token into the latest assistant bubble.
 export function AskPetal({ suggestionId, explanation }: Props) {
+  const t = usePack()
   // Opening bubble starts empty (caret-only) and fills with the Mandarin
   // translation once it lands; `seeding` drives that loading caret.
   const [messages, setMessages] = useState<ChatMessage[]>([{ role: 'assistant', content: '' }])
@@ -137,7 +139,7 @@ export function AskPetal({ suggestionId, explanation }: Props) {
           ref={inputRef}
           value={input}
           onChange={(e) => setInput(e.target.value)}
-          placeholder="Ask why… / 问为什么…"
+          placeholder={t.editor.askPlaceholder}
           className="min-w-0 flex-1 rounded-full px-3 py-1.5 text-xs focus:outline-none"
           style={{
             background: 'var(--color-surface-alt)',