feat(frontend): balance-sync tests + optimistic updates, SR live region, onboarding reducer extraction

[david87131]

Summary

Four small frontend changes across the Real-time Balance Sync and Onboarding Progress Tracker modules, bundled into one PR.

• [Frontend] Add unit tests for Real-time Balance Sync #805 — unit tests for Real-time Balance Sync (src/hooks/useBalanceSync.test.ts): added coverage for the Horizon address path (with native→XLM normalisation), the disabled state, the new optimistic update, and the isStale flag.
• [Frontend] Improve screen reader support for Real-time Balance Sync #806 — screen reader support for Real-time Balance Sync (src/components/SupportPanel.tsx): added a single authoritative visually-hidden role="status" live region that announces sync state ("Syncing…", "unfunded", "Balance synced: X CODE"), and made the visual balance row non-live so screen readers hear one clear message instead of raw element swaps.
• [Frontend] Enable optimistic updates in Real-time Balance Sync #807 — optimistic updates in Real-time Balance Sync (src/hooks/useBalanceSync.ts): added applyOptimistic(code, balance) so a just-submitted balance can be reflected immediately; the next poll reconciles it with the authoritative value.
• [Frontend] Refactor state logic for Onboarding Progress Tracker #808 — refactor state logic for the Onboarding Progress Tracker (src/components/onboarding-reducer.ts): extracted the reducer, its OnboardingState/OnboardingAction types and an initial-state factory out of the component into a pure module, decoupling state logic from the view and making it independently unit-testable. Component behaviour is unchanged.

Test plan

• vitest run src/hooks/useBalanceSync.test.ts src/components/onboarding-reducer.test.ts — 12 tests pass (6 balance-sync + 6 onboarding-reducer)
• tsc --noEmit — no type errors in any of the changed files

Note (pre-existing, not introduced by this PR)

The repository's React component test environment fails on main with a jsdom appendChild error (verified on the unmodified tree), so component-render tests for SupportPanel/OnboardingProgressTracker cannot run locally. The work above is therefore covered by hook tests and a pure reducer test, which do run green; tsc confirms the component edits type-check.

Closes #805
Closes #806
Closes #807
Closes #808

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