fix(docs): add explicit cleanup hooks for playground timeouts

[legend4tech]

Fixes #155: Memory Leak Prevention on Interactive Playground Tabs

Description

This PR resolves a memory leak issue and prevents "unmounted component" state update errors within the Developer Gateway & Docs (src/app/docs/page.tsx). Previously, repeatedly switching between documentation tabs or navigating away from the page while the handleCopy or handleTestInvoke timers were still running resulted in an accumulation of unmanaged listeners (timeouts).

Technical Changes

• Explicit Cleanup Hooks: Added an explicit useEffect cleanup hook that destroys any active timers when the module unmounts, severing pending context subscriptions.
• Reference Tracking: Utilized useRef (copyTimeoutRef and invokeTimeoutRef) to accurately track and maintain the background timers without triggering unnecessary re-renders.
• Deduplication: Added checks to immediately sever any currently running timeouts if a user rapidly spams the interaction buttons before triggering a new timeout.

Verification

• Rapidly switching between the Rust/JS tabs while an action is processing no longer causes overlapping UI bugs.
• Navigating away from the /docs route while an RPC invocation is processing no longer throws React state update warnings in the console.

closes #155

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