Refactor shared frontend workflows

[PengfanZ]

What changed

• extracted browser-storage error handling and cross-tab synchronization into reusable utilities
• consolidated the shared lifecycle for the language and currency dropdowns
• moved snapshot and Live sharing orchestration out of App.tsx into a focused hook
• memoized only the derived activity data and dashboard calculations that perform array scans
• added direct tests for storage failures, unchanged writes, and unsupported values

Why

App.tsx and several persistence/menu components repeated the same infrastructure logic. Centralizing those responsibilities makes the frontend easier to read and change while preserving the existing UI, storage keys, URL formats, analytics, and Supabase contracts.

User impact

No intended user-facing behavior change. Local persistence, cross-tab updates, currency/language menus, snapshot sharing, Live sharing, and responsive layouts retain their existing behavior.

Validation

• npm run typecheck
• npm run lint
• npm run test:coverage — 283 tests, 100% statements/branches/functions/lines
• npm run test:backend — 121 pgTAP tests
• npx supabase db advisors --local --type all --level warn --fail-on error — no issues
• npm run test:e2e — 13 Playwright tests
• npm run build:pages
• browser QA: language, currency, expense creation, snapshot QR, reload persistence, mobile menu/Escape focus, and clean console