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.

internal/suggestions/chat.go

@@ -40,14 +40,17 @@ func (h *Handler) chat(w http.ResponseWriter, r *http.Request) {
 		original, replacement, explanation, typ string
 		fromPos                                 int
 		contentText                             string
+		pairLang                                string
 	)
 	err := h.DB.QueryRow(
-		`SELECT s.original, s.replacement, s.explanation, s.type, s.from_pos, d.content_text
+		`SELECT s.original, s.replacement, s.explanation, s.type, s.from_pos, d.content_text,
+		        COALESCE(u.pair_lang, '')
 		   FROM suggestions s
 		   JOIN documents d ON d.id = s.doc_id
+		   JOIN users u ON u.id = d.user_id
 		  WHERE s.id = ? AND d.user_id = ?`,
 		sugID, auth.UserID(r.Context()),
-	).Scan(&original, &replacement, &explanation, &typ, &fromPos, &contentText)
+	).Scan(&original, &replacement, &explanation, &typ, &fromPos, &contentText, &pairLang)
 	if errors.Is(err, sql.ErrNoRows) {
 		httputil.ErrorJSON(w, http.StatusNotFound, "suggestion not found")
 		return
@@ -58,7 +61,7 @@ func (h *Handler) chat(w http.ResponseWriter, r *http.Request) {
 	}
 
 	paragraph := surroundingParagraph(contentText, fromPos)
-	systemPrompt := llm.AskPetalSystemPrompt(original, replacement, typ, explanation, paragraph)
+	systemPrompt := llm.AskPetalSystemPrompt(original, replacement, typ, explanation, paragraph, llm.LangFor(pairLang))
 
 	// SSE requires an unbuffered, flushable writer. chi's middleware writers pass
 	// Flush through; bail with a plain error if somehow they don't.