fix: prevent state updates on unmounted component in useStreams

[viccoder-oops]

Summary

• Adds a mountedRef guard in useStreams to prevent calling setStreams, setError, and setLoading after the component unmounts
• Marks mountedRef.current = false in the useEffect cleanup alongside clearInterval
• Eliminates the React warning "Can't perform a React state update on an unmounted component" that occurred when the polling interval fired after navigation away from the dashboard

Changes

• src/hooks/useStreams.ts: added mountedRef boolean guard around all state setters inside the async fetch callback; cleanup function now sets mountedRef.current = false before clearing the interval

Closes #4