fix: pause live feeds on hidden tabs

[kpassito]

Summary

• add a shared page visibility hook with SSR and unsupported-API fallback
• pause WebSocket subscriptions and reconnect timers while the page is hidden, then reconnect/revalidate when visible again
• gate dashboard polling and WebSocket merges behind page visibility and show a passive PAUSED badge

Verification

• npx eslint src/app/hooks/usePageVisibility.ts src/app/hooks/useSocket.ts src/app/components/providers/SocketProvider.tsx src/app/components/PriceFeedCard.tsx
• npm run build (fails before reaching this change because of existing unrelated errors in src/app/components/FloatingSidebar.tsx and src/app/logs/page.tsx)

Closes #282

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