chore: add mobile platform code owners

[adonesky1]

Explanation

Add @MetaMask/mobile-platform as a repository-wide code owner so the team can review dapp connection UI-related changes. Preserve @MetaMask/wallet-integrations ownership during the transition.

References

Requested during review coordination for the dapp connection UI improvements.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes