fix: remaining audit findings — performance and visual consistency

[laurentiu021]

Summary

Final batch of audit findings (Low priority):

• NetworkSharedState — TracerouteHops converted from ObservableCollection to BulkObservableCollection with ReplaceWith(). Eliminates per-hop UI change notifications during route refreshes.
• ServicesViewModel — safety level counts (Safe/Caution/Critical) now computed in a single foreach pass instead of 3 separate LINQ .Count() queries over 200+ items.
• DnsHostsView — removed stale HorizontalGridLinesBrush="#1E293B" property (no visual effect since GridLinesVisibility="None").
• AppBlockerView — DataGrid BorderThickness set to 0, matching all other DataGrids in the app.
• ConsoleViewModel RemoveAt(0) — left as-is (O(1) in practice with MaxLines trim of 1 item; no WPF-bindable deque alternative).

Test plan

• Traceroute tab: route updates display correctly without flicker
• Services tab: filter works, safety counts correct
• DNS & Hosts tab: table displays correctly (no border artifacts)
• App Blocker tab: DataGrid blends with dark background

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d7a3c94d-6399-46cb-8a7a-408dcf93825c

📥 Commits

Reviewing files that changed from the base of the PR and between 760dd23 and 21b2e74.

📒 Files selected for processing (6)

• CHANGELOG.md
• SysManager/SysManager/SysManager.csproj
• SysManager/SysManager/ViewModels/NetworkSharedState.cs
• SysManager/SysManager/ViewModels/ServicesViewModel.cs
• SysManager/SysManager/Views/AppBlockerView.xaml
• SysManager/SysManager/Views/DnsHostsView.xaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/audit-low-priority

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