ui(extensions): Updates tab must not show false “up to date”

[ZhuchkaTriplesix]

Parent

Part of epic #490. Motion shell for Extension Manager landed in #488/#489; product tab is still a stub.

Problem

Updates tab always shows “All installed extensions are up to date!” — no check, and can show success while Installed/Marketplace still loading.

Proposal

Until live check-updates API (0.5.0):

• Gate on _loading, and/or
• Show honest empty/disabled copy (“Update checks coming soon”), and/or
• Hide/disable the Updates tab

Do not claim success without a scan.

Files

• lib/features/extensions/presentation/pages/extension_manager_dialog.dart

Acceptance

• No false “up to date” while loading or without a real check
• TabStrip / CrossFadeStack behavior unchanged

[ZhuchkaTriplesix]

Shipped on dev (PR targeted dev, auto-close did not fire).