[975] Frontend: Implement account risk summary card with actionable warnings

[UzyKhs]

Summary

• Extracts the inline RiskSummaryCard from VaultDashboard.tsx into a presentational frontend/src/components/RiskSummaryCard.tsx that receives all copy via props, exposes itself as a labelled role="region", and announces warning changes via aria-live="polite".
• Adds an optional healthyAction CTA to the all-clear state; the dashboard passes a "Compare strategies" action that navigates to /compare.
• Moves all risk-summary copy to i18n under vaultDashboard.riskSummary.* in en.ts and mirrors it with Spanish translations in es.ts; the XLM-fee warning CTA label changes from "Adjust amount" to "Review deposit".
• Adds RiskSummaryCard.test.tsx covering warning rendering + CTA click handlers, the labelled region, the all-clear state with/without the healthy CTA, and singular vs. plural badge labels.
• Documents the signals and CTAs in docs/frontend/account-risk-summary.md.

Closes #975

Test plan

• npm run test:run -- src/components/RiskSummaryCard.test.tsx src/components/VaultDashboard.test.tsx src/components/VaultDashboard.emptystate.test.tsx — 3 files, 26 tests, all passing
• Manually verify the dashboard's risk card in healthy and warning states, including the "Compare strategies" and "Review deposit" CTAs
• Switch locale to Spanish and confirm the risk summary copy is translated