Mirrors the backend schema additions (StepRowBlock.items.subtitle,
new IconBlock) needed for the /lebensuhr Pages migration: PageBlock
type + mapPayloadPageBlock in payload.ts, a "icon" case in
PageBlocks.tsx, and SYMBOLIC_ICONS (the clock face) added alongside
STEP_ICONS in StepIcons.tsx — a distinct registry since these icons
are standalone, not part of an icon-above-text row.
/lebensuhr, /3x3-system, and /7-tage-klarheits-check each defined
their own local copies of the same hand-drawn brand-line icons.
Moved them into app/components/icons/StepIcons.tsx as a keyed
registry (STEP_ICONS) — also what the new Payload Pages collection's
StepRowBlock select field maps onto by key, so a page-builder step
row can reference these same icons without duplicating SVG paths a
third time.
Marco