Add dark mode, mobile layout, toasts, and query cache

[miss-yusrah]

Summary

Frontend UX foundations for YieldVault: consistent theming, usable mobile layout, user-facing toasts, and a real data cache.

#1271 Dark mode

• Settings already had Light / Dark / System; that control is wired to a shared theme layer.
• Theme tokens live in frontend/src/styles/theme.css (CSS variables for surfaces, text, accents, toasts). Light mode overrides [data-theme="light"].
• ThemeProvider + PreferencesProvider persist the preference (light | dark | system) in localStorage (yieldvault-theme), not only the resolved color.
• System preference is detected on load and kept in sync via prefers-color-scheme (usePrefersColorScheme / subscribeToSystemTheme).
• FOUC script in index.html applies the saved or OS theme before paint.
• Navbar toggle, Settings cards, and several hardcoded colors now use theme variables.

#1274 Responsive layout

• Breakpoints and touch rules in frontend/src/styles/responsive.css (375 / 768 / 1024, 44px hit targets, safe-area insets).
• Mobile-first nav: hamburger on small screens, slide-over menu, overlay, Escape to close, body scroll lock, nav-mobile-link rows.
• Settings locale grid stacks on phones; tables/toasts/panels pick up existing and new mobile rules.
• accessibility.css is now imported (coarse-pointer / focus styles).

#1278 Toast notifications

• Added sonner.
• ToastProvider is the notification context (success / error / warning / info).
• Auto-dismiss (duration) and manual dismiss (close button).
• Queue capped at 5 (oldest dropped); dedupe window kept.
• Alias: NotificationProvider / useNotification.
• Toaster theme follows data-theme.

#1279 React Query cache

• TanStack Query was already in the app; cache behavior is now explicit.
• Stale-while-revalidate: staleTime + placeholderData keep previous data while refetching.
• networkMode: "offlineFirst" so cached data is used offline.
• localStorage persistence via persist client (yieldvault-query-cache, 24h), skipped in tests.
• Invalidation helpers + useInvalidateQueries.
• Optimistic deposit/withdraw updates remain in useVaultMutations / optimisticVaultCache.

Test plan

• cd frontend && npm install
• Settings → Appearance: Light, Dark, System; reload; change OS theme while on System
• Toggle theme from the navbar; confirm charts, settings, errors, and toasts follow tokens
• 375 / 768 / 1024: hamburger vs desktop links, overlay, 44px targets, no horizontal overflow
• Trigger success/error/warning/info toasts; auto-dismiss and close; queue more than 5
• Deposit/withdraw still optimistic; disconnect network and confirm cached vault/balance data; reconnect refetches

closes #1271
closes #1274
closes #1278
closes #1279

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