Fix translatable-field collection for repeater groups and tab-organized nested forms

[AIC-BV]

Found while production-testing this branch as requested in #115. Two issues stopped translatable: true fields from being picked up with real-world field definitions:

1. Repeater/blocks groups sharing a field name lose all but the first group. getAutoTranslatableFields() merged group fields with a keyed union (+=). Block groups routinely all wrap their content in an identically-named nestedform (e.g. data), so only the first group's fields were scanned and every other block silently fell back to plain copy. Groups are now scanned separately.
2. form.tabs.fields / form.secondaryTabs.fields were not descended into, so nested forms that organize fields in tabs never had their fields collected.

Both verified end-to-end on our production field definitions (blocks repeater, nl-BE → en via Google) and covered by regression tests (suite green: 18 tests, 27 assertions).

🤖 Generated with Claude Code

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