DS::SelectableCard — a checkbox rendered as a selectable card (whole card
toggles; brand-accent border + bg-surface when selected via peer-checked
on the sibling). Submits like a normal checkbox, so the bucket's
replace-all form is unchanged. Lookbook preview + component test.
Retirement bucket now renders each account as a DS::SelectableCard
(name · type · balance) instead of a bare checkbox row. Money stays
privacy-sensitive.
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