Remove the "Type" header from the New Item sheet

The first step of the New Item sheet (CreateItemPickerView) opens with a step header: a plus icon, the title "Type", and "Choose what kind of item to add to your vault." It adds nothing. The navigation title already says "New Item", and the Code / Note / Recovery Phrase cards below it speak for themselves.

Change

• Remove the DetailEditorStepHeader section from CreateItemPickerView, so the sheet goes straight to the three option cards under the "New Item" title.
• Check the sheet still sizes to fit (reportsFittedSheetHeight()), with sensible top spacing above the first card, and that picking an item still slides on to its first step.
• Update the view's doc comment, which describes it as "with a step header".

Done when

• The header is gone, and the sheet looks right in light and dark.
• CreateItemPickerViewSnapshotTests are re-recorded.