feat: implement asynchronous weather sync with status polling

[hoanhtnb]

Summary

Performance & Responsiveness

• Non-blocking weather sync trigger: Power button and Home long-press now trigger weather/time sync via background task instead of blocking UI loop.
• Home sync status polling: Home header now polls async sync state/result and updates status message without freezing navigation.
• Sleep safety during sync: Main loop now treats active background sync as activity to prevent auto-sleep interrupting in-flight weather sync.

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AI Usage

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helps set the right context for reviewers.

Did you use AI tools to help write this code? **YES **