Fix i18n-translate.ts truncation on large batches

scripts/i18n-translate.ts issues a single Claude request capped at ~4096 output tokens, so large key batches get truncated/dropped. Surfaced during the full web string sweep (1004 keys) — the run had to use a throwaway chunked translator. Fix: chunk the keys into multiple requests (or raise max_tokens + paginate) and reassemble, keeping i18n:check parity as the gate. Low priority but bites anyone adding a big batch of strings or a new locale.