Fix the deeply nested cards in the project form (screenshot: Cover Media >
Cover > preview card, with duplicate labels):
- MediaFieldPicker no longer wraps itself in AppCards. It renders one flat
bordered box laid out in two columns (preview | controls), dropping two
nested card layers and the duplicate title.
- Cover Media and Assets now sit side by side in a two-column grid instead
of stacked with a separator.
All 375 tests pass; tsc and eslint clean.