Refactor router state to more closely match tab bar behavior

[72A12F4E]

What changed?

This change modifies NavigationState to separately track the tab's selection behavior from the selected tab.

Why did it change?

This should prevent the issue where switching back & forth between messages / other tabs on a phone will leave the app showing the "Select a conversation" screen. This also simplified the Bindings across the root screens for each tab.

How is this tested?

Screenshots/Videos (when applicable)

Checklist

• My code adheres to the project's coding and style guidelines.
• I have conducted a self-review of my code.
• I have commented my code, particularly in complex areas.
• I have made corresponding changes to the documentation.
• I have tested the change to ensure that it works as intended.

[72A12F4E]

@garthvh I haven't been able to give this a full test cycle yet, but would appreciate a quick look over the changes nonetheless

[garthvh]

Seems better than ever being down an extra level, worked better than the current routing for me, bluetooth link in the new live activity still works.

[garthvh]

Looks good and previews work again, I approve