fix: improve mobile nav toast and error boundary

[devfoma]

Summary

• improve mobile nav tap targets, focus rings, and icon-only accessible labels
• bound and deduplicate toast queues with stable ids and an aria-live container
• make the error boundary fallback safer and retry immediately without exposing raw error text by default

Closes #281
Closes #283
Closes #284

Tests

• npm test -- src/tests/shared/error-boundary.test.tsx src/tests/shared/toast.test.tsx src/tests/layout/mobile-nav.test.tsx (blocked locally: dependencies are not installed; vitest not found)
• npm run typecheck (blocked locally: dependencies are not installed; tsc not found)

[netlify]

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