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/collocation.go

@@ -18,10 +18,11 @@ const CollocationInterval = 25 * time.Second
 // reflect the full piece. Each flag carries a native replacement to apply.
 //
 // The tone argument is accepted for a uniform pass signature and passed through
-// to the prompt so a hint can prefer a register-appropriate pairing.
-func RunCollocation(ctx context.Context, client LLMClient, contentText, tone string) ([]RawSuggestion, error) {
+// to the prompt so a hint can prefer a register-appropriate pairing. `lang` is
+// the writer's pair language — the one each hint's short gloss is written in.
+func RunCollocation(ctx context.Context, client LLMClient, contentText, tone string, lang Lang) ([]RawSuggestion, error) {
 	raw, err := client.Complete(ctx, CompletionRequest{
-		Messages:          CollocationMessages(contentText, tone),
+		Messages:          CollocationMessages(contentText, tone, lang),
 		MaxTokens:         2048,
 		Temperature:       0.3,
 		RepetitionPenalty: 1.15,