FCE-2467: Implement proper crypto polyfil

[MiloszFilimowski]

Description

• use react-native-get-random-values as getRandomValues.

Motivation and Context

• We had a temporary implmenation.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[linear]

FCE-2467 Implement proper crypto polyfil

[Copilot]

Pull request overview

This PR replaces a temporary Math.random()-based implementation of crypto.getRandomValues with the proper react-native-get-random-values library polyfill.

• Adds react-native-get-random-values dependency to provide secure random value generation
• Removes the temporary crypto polyfill implementation and associated TODO comment

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/mobile-client/package.json	Adds react-native-get-random-values dependency
yarn.lock	Updates lock file with new dependency and its transitive dependency (fast-base64-decode)
packages/mobile-client/src/webrtc-polyfill.ts	Imports react-native-get-random-values and removes temporary Math.random()-based crypto implementation

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