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/llm/lang_test.go

@@ -0,0 +1,85 @@
+package llm
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLangForFallsBackToDefault(t *testing.T) {
+	if got := LangFor("zh"); got.Code != "zh" {
+		t.Fatalf("LangFor(zh) = %+v", got)
+	}
+	if got := LangFor("pt-PT"); got.Code != "pt-PT" {
+		t.Fatalf("LangFor(pt-PT) = %+v", got)
+	}
+	// A blank column, a stray value, and stray whitespace all resolve rather
+	// than erroring — a prompt is the wrong place to discover a config problem.
+	for _, in := range []string{"", "  ", "klingon", "ZH"} {
+		if got := LangFor(in); got.Code != DefaultLang.Code {
+			t.Fatalf("LangFor(%q) = %q, want the default %q", in, got.Code, DefaultLang.Code)
+		}
+	}
+	if got := LangFor(" zh "); got.Code != "zh" {
+		t.Fatalf("LangFor with padding = %+v", got)
+	}
+}
+
+// The three prompts that name the writer's language must actually name *hers*.
+// Before Phase 19 all three said "Simplified Chinese" outright, which is the
+// bug this guards: a pt-PT writer asking "porquê" would have been answered in
+// Mandarin.
+func TestPromptsNameTheWritersLanguage(t *testing.T) {
+	pt := LangFor("pt-PT")
+
+	collocation := CollocationMessages("The rain was strong.", "casual", pt)[0].Content
+	if !strings.Contains(collocation, "European Portuguese") {
+		t.Fatalf("collocation prompt doesn't ask for a pt-PT gloss:\n%s", collocation)
+	}
+	if strings.Contains(collocation, "Simplified Chinese") {
+		t.Fatalf("collocation prompt still hardcodes Chinese:\n%s", collocation)
+	}
+	// The tone steering must survive alongside the language — they share one
+	// format string, and getting the verbs in the wrong order silently drops one.
+	if !strings.Contains(collocation, "relaxed, friendly, and conversational") {
+		t.Fatalf("collocation prompt lost its tone guidance:\n%s", collocation)
+	}
+
+	translate := TranslateMessages("Try a shorter sentence here.", pt)[0].Content
+	if !strings.Contains(translate, "European Portuguese") || strings.Contains(translate, "Chinese") {
+		t.Fatalf("translate prompt targets the wrong language:\n%s", translate)
+	}
+
+	ask := AskPetalSystemPrompt("origin", "replacement", "grammar", "explanation", "paragraph", pt)
+	if !strings.Contains(ask, "European Portuguese") || strings.Contains(ask, "Mandarin") {
+		t.Fatalf("ask-petal prompt targets the wrong language:\n%s", ask)
+	}
+	if !strings.Contains(ask, "porquê") {
+		t.Fatalf("ask-petal prompt doesn't recognise her word for \"why\":\n%s", ask)
+	}
+	// The suggestion context is positional in that template; a mis-numbered
+	// verb would quietly blank one of these fields.
+	for _, want := range []string{"origin", "replacement", "grammar", "explanation", "paragraph"} {
+		if !strings.Contains(ask, want) {
+			t.Fatalf("ask-petal prompt dropped %q:\n%s", want, ask)
+		}
+	}
+	if strings.Contains(ask, "%!") {
+		t.Fatalf("ask-petal prompt has a formatting error:\n%s", ask)
+	}
+}
+
+// The zh pair is in daily use and must be untouched by the extraction: its
+// prompts should read exactly as they did when they were hardcoded.
+func TestDefaultPairStillReadsAsBefore(t *testing.T) {
+	zh := LangFor("zh")
+
+	if got := CollocationMessages("x", "", zh)[0].Content; !strings.Contains(got, "Simplified Chinese (Mandarin) gloss in parentheses") {
+		t.Fatalf("zh collocation gloss changed:\n%s", got)
+	}
+	if got := TranslateMessages("x", zh)[0].Content; !strings.Contains(got, "natural, friendly Simplified Chinese (Mandarin)") {
+		t.Fatalf("zh translate target changed:\n%s", got)
+	}
+	if got := AskPetalSystemPrompt("a", "b", "c", "d", "e", zh); !strings.Contains(got, "为什么") {
+		t.Fatalf("zh ask-petal lost its Mandarin \"why\":\n%s", got)
+	}
+}