Preserve nav state across MainActivity recreation

[MihaiCristianCondrea]

Summary

• gate NavController graph initialization so we only set the graph once per process start
• retain and reuse the existing controller when UI state updates so label visibility and listeners refresh without rebuilding the stack
• navigate to new preferred start destinations with a controlled popUp and persist the nav state across recreation

Testing

• ./gradlew test (fails: Android SDK not available in CI container)

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https://chatgpt.com/codex/tasks/task_e_68cd14979bec832dae7f684fe0273eda