Fixes blank screen if the user chooses to Connect Now during onboarding

[myleshorton]

See https://wdynhnkxvsdx.slack.com/archives/C04T8M4AB1Q/p1770411785853549

This pull request improves the user experience and stability of the onboarding and system extension dialogs, particularly on macOS. The main focus is on deferring navigation actions to the next frame to prevent calling navigation or state changes during widget builds, which can cause runtime errors or unexpected behavior.

Improvements to navigation and dialog handling:

• Defers the closing of the system extension dialog until the next frame after detecting the extension is installed and activated, preventing navigation during the build process (macos_extension_dialog.dart).
• Defers the presentation of the macOS extension dialog until the next frame after onboarding is completed, ensuring navigation actions are not performed during build (onboarding.dart).
• Removes redundant appRouter.pop() call before calling onboardingCompleted() to streamline the onboarding completion logic (onboarding.dart).

Stability improvements in effect hooks:

• Wraps routing mode logic in a microtask using Future(() { ... }) to ensure it runs after the current build, avoiding side effects during widget construction (onboarding.dart). [1] [2]

[jigar-f]

Thanks for the fixes.