a11y: add ARIA attributes to explainer cards and CTA banner

[GeneralJerel]

Description

Severity: P3 — Accessibility

Two components lack proper accessibility attributes:

Explainer cards (`explainer-cards.tsx`)

• No semantic landmark, no heading hierarchy
• Icon SVGs lack `aria-hidden="true"`
• Screen readers will announce raw SVG path data
• Cards are invisible to assistive technology structure

CTA banner (`page.tsx:50-54`)

• SVG icon has no `aria-hidden` or accessible label
• The `` link has no `aria-label` beyond its text content (minor — "Get started" text is sufficient)

Suggested fix

• Add `aria-hidden="true"` to all decorative SVGs
• Wrap explainer cards in a `
  ` with an appropriate heading or `aria-label`

From self-review findings F11 and F12