Disable request initiator panel in NetworkItemView

[huntie]

Summary

Follow up to #154. We don't yet support CDP request initiators (JS call stack) in our Network panel V1.

Test plan

Before

Screen.Recording.2025-06-29.at.13.47.40.mov

⚠️ All sub-panels populated except for "Initiator"

After

✅ Sub-panel is disabled

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.