fix(auth): wire heroImage prop into AuthLayout hero section

[extolkom]

closes #754

PR Description
Title: [Tech-debt] Wire heroImage prop into AuthLayout hero section
Summary
AuthLayout.tsx declared a heroImage prop and destructured it, but immediately discarded it with void heroImage, rendering a hardcoded background instead. This made the component's public API misleading — callers could pass a heroImage and reasonably expect it to be used, but it had no effect.
Changes

Wired heroImage into the hero section using next/image, so the prop now actually controls the rendered image.
Removed the void heroImage suppressant now that the prop is genuinely used.
Updated all call sites passing heroImage to AuthLayout to confirm they render as expected with the new behavior.
Updated any stories/tests referencing the heroImage prop to reflect the real rendering instead of the previous no-op.

Why
Keeps the component's interface honest — a prop that's accepted should affect output. Removing dead suppressants also avoids masking future unused-variable issues introduced by mistake.
Testing
Verified locally that passing different heroImage values updates the rendered hero image as expected, and that omitting it falls back sensibly. Ran type-check and existing test/story suite — no unused-variable errors, all green.
Acceptance criteria met

✅ heroImage is rendered in the hero section via next/image
✅ No void propName suppressants remain in the component
✅ TypeScript compilation shows no unused-variable errors

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