fix: replace module-scoped ref registry with instance-local ref in UIKit Overflow component

Summary

The Overflow component in app/containers/UIKit/Overflow.tsx uses a module-scoped touchable map to store React.RefObject instances keyed by blockId.

Problem

• Ref leaks: refs stored in the module-level map are never removed, so every blockId seen during the app session accumulates in memory.
• Wrong anchor: when blockId is empty (falls back to ''>) or repeats across different component instances, multiple Overflow components share the same ref, causing the Popover to anchor to the wrong element.

Suggested Fix

Replace the global map with an instance-local useRef inside the Overflow component, tying the ref lifecycle to the component instance:

// Before (module-scoped, leaks & collides)
const touchable: { [key: string]: React.RefObject<View | null> } = {};

export const Overflow = ({ element, loading, action, parser }: IOverflow) => {
  const blockId = element?.blockId || '';
  if (!touchable[blockId]) {
    touchable[blockId] = React.createRef();
  }
  const touchableRef = touchable[blockId] as React.RefObject<any>;
  // ...
};

// After (instance-local, safe)
export const Overflow = ({ element, loading, action, parser }: IOverflow) => {
  const touchableRef = useRef<React.ElementRef<typeof Touch>>(null);
  // ...
};

References

• Flagged in PR #6875: https://github.com/RocketChat/Rocket.Chat.ReactNative/pull/6875#discussion_r2988571189
• Requested by @diegolmello