system/ui: performance optimizations for WiFi Manager with cached network data

[deanlee]

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This PR fixes the FPS issue where the UI could not reach 60 FPS due to inefficient network list retrieval. Previously, _run_coroutine_sync was called at 60Hz to fetch network lists, causing the UI thread to block while waiting for the asyncio thread to respond.

Now, by implementing an event-based approach with callbacks, the network list is only updated when actual changes occur. This significantly improves performance:

• CPU usage reduced from ~55% to ~15% during UI rendering on my laptop
• Frame rate consistently reaches 60 FPS instead of stuttering

Key improvements:

1. Event-based updates: Added a new networks_updated callback to notify the UI when network data changes
2. Cached network information: Store network data locally in the WifiManagerUI
3. Embedded network status: Added is_saved field to NetworkInfo class to avoid repeated checks

These changes maintain the same functionality while significantly reducing CPU load and making the UI more responsive.

_run_coroutine_sync is preserved in the codebase as it remains valuable for other direct DBus operations and for future advanced network UI widgets that need synchronous access.