fix: prefer close over back in iframe header

[broody]

Summary

• make iframe/popup header default to close instead of back
• keep explicit force-back behavior unchanged for screens that opt in
• scope this PR to header navigation behavior only

Test plan

• Run repo hooks/tests via commit hook
• Open popup flows (connect/starterpack/update-session) and verify close icon is shown instead of back

[vercel]

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Project	Deployment	Actions	Updated (UTC)
controller-example-next	Ready	Preview	Feb 25, 2026 1:14am
keychain	Ready	Preview	Feb 25, 2026 1:14am
keychain-storybook	Ready	Preview	Feb 25, 2026 1:14am

[claude]

Claude finished @broody's task —— View job

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Documentation Analysis Complete

• Analyze the code change to understand user-facing impact
• Search docs repository for related content
• Determine if documentation updates are needed
• Update docs if necessary (or explain why not needed)

Summary

No documentation updates needed.

This PR changes internal UI behavior in the NavigationHeader component (preferring "close" over "back" in iframe/popup contexts). This is an internal keychain implementation detail that:

• Does not affect any documented APIs or configuration options
• Is not referenced anywhere in the user-facing documentation
• Represents UI polish rather than a feature or API change
• Falls under the "DEFAULT TO NO CHANGES" rule for internal refactors and UI improvements

The documentation focuses on API usage, configuration, and session policies - not on internal keychain UI component behavior. Similar to changing button colors or layouts, this type of UI refinement doesn't require documentation updates.

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