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/translate.go

@@ -25,14 +25,15 @@ type translateResponse struct {
 func (h *Handler) translate(w http.ResponseWriter, r *http.Request) {
 	sugID := chi.URLParam(r, "id")
 
-	var explanation string
+	var explanation, pairLang string
 	err := h.DB.QueryRow(
-		`SELECT s.explanation
+		`SELECT s.explanation, 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(&explanation)
+	).Scan(&explanation, &pairLang)
 	if errors.Is(err, sql.ErrNoRows) {
 		httputil.ErrorJSON(w, http.StatusNotFound, "suggestion not found")
 		return
@@ -48,7 +49,7 @@ func (h *Handler) translate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	out, err := llm.RunTranslate(r.Context(), h.Client, explanation)
+	out, err := llm.RunTranslate(r.Context(), h.Client, explanation, llm.LangFor(pairLang))
 	if err != nil {
 		httputil.ErrorJSON(w, http.StatusBadGateway, "translate failed: "+err.Error())
 		return