perf(ui): reduce unnecessary UI-thread work

[Neutron-0]

This PR reduces unnecessary UI-thread work identified while investigating Issue #133.

Summary

This PR reduces unnecessary UI‑thread work while preserving existing behavior. It addresses performance concerns raised in GitHub issue #133 by tightening log handling, pausing the dashboard uptime timer when inactive, and avoiding redundant tray‑icon refreshes.

Changes

• Log buffering – Introduced a StringBuilder in MainViewModel and rebuilt the buffer only when the log line queue exceeds the UI limit.
• Dashboard uptime timer – Timer start is now deferred; SetUptimeTimerEnabled starts the timer when telemetry projection is enabled and stops it when disabled.
• Tray icon caching – Added cached fields for all displayed metrics, implemented early‑return when values are unchanged, and updated caches after a UI refresh.
• Allocation reduction – Fewer string allocations and dispatcher posts across the UI.
• Code cleanup – Removed a duplicate cache‑update block in TrayIconService.

Behavior

No functional changes are intended. All UI elements continue to display the same information; only the amount of work performed on the UI thread is reduced.

Testing

• dotnet build OmenCore.sln succeeds with 0 warnings and 0 errors.
• All existing unit tests (OmenCoreApp.Tests) pass.
• Manual verification confirms:
  • Log view updates correctly after appending and after buffer reload.
  • Dashboard uptime counter updates only when the dashboard is visible.
  • Tray tooltip, menu, and badge updates occur only when a displayed value actually changes.

Expected Impact

• Lower CPU usage on the UI thread during idle periods.
• Fewer allocations and garbage‑collection pressure.
• Reduced dispatcher activity leading to smoother UI responsiveness, especially when the app is monitoring continuously.

Related to #133