feat: improve dapp accessibility

[snkk2x-collab]

Summary

• add a keyboard-visible skip link and main landmark to DappShell
• add explicit accessible names and busy state to wallet connection controls
• announce wallet auth errors with role="alert" and aria-live="polite"
• expose tier and score progress through screen-reader labels and progressbar semantics
• document the dapp accessibility checklist in Docs/accessibility.md

Validation

• pnpm build
• simulated merge with origin/main completed without conflicts
• pnpm build also passed after the simulated merge

Notes

tsc --noEmit still reports existing issues in components/api-response-card.tsx and lib/wallet-kit.ts. Next build also reports an existing @stellar/freighter-api dynamic import warning after merging current main, but exits successfully.