Make FirstDepositStep's asset-selection cards keyboard-operable

[Abidoyesimze]

Summary

Same pattern as the wallet-selection cards: plain div onClick for asset choice (XLM/USDC/EURC), with no role, tabIndex, or key handler.

Affected files

• src/components/onboarding/steps/FirstDepositStep.tsx

Tasks

• Add role="button", tabIndex={0}, and an Enter/Space key handler to the asset-option cards.
• Verify Tab order reaches every asset option and Enter/Space activates selection.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

a11y, onboarding, bug

Notes

Sourced from the 2026-07-25 round-2 follow-up audit of the frontend codebase at commit 779e531, run after the first 60-issue audit batch (#525-#584) was substantially resolved. Verified against current file content at time of writing.