Ensure null-safe Firebase auth handling

[maxschwinghammer]

Summary

• Avoid scheduling notifications when no user is logged in
• Redirect unauthenticated users to the sign-in screen on launch

Testing

• ./gradlew test (fails: Gradle daemon started but tests did not run due to environment constraints)

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https://chatgpt.com/codex/tasks/task_b_68a70fa0fba88327887a7e3ad8acfa98

[Copilot]

Pull Request Overview

This PR implements null-safe Firebase authentication handling to prevent crashes and ensure proper user flow. The changes ensure notifications are only scheduled for authenticated users and redirect unauthenticated users to the sign-in screen.

• Move Firebase auth check from class property to local variable for thread safety
• Add null check before scheduling notifications to prevent crashes
• Implement redirect logic for unauthenticated users using LaunchedEffect

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[maxschwinghammer]

@copilot open a new pull request to apply changes based on the comments in this thread

[Copilot]

@maxschwinghammer I've opened a new pull request, #231, to work on those changes. Once the pull request is ready, I'll request review from you.