refactor: final modernization stragglers (seal, ConfigureAwait, ToArray)

[laurentiu021]

Summary

Closes out the last verified modernization stragglers from the completeness audit. refactor:, behavior-preserving, no release.

Changes

• seal BulkObservableCollection — no subclasses exist; sealing matches the rest of the codebase and allows JIT devirtualization.
• ConfigureAwait(false) on the three service awaits that actually await (SpeedTestService Task.WhenAll, PingMonitorService + TracerouteMonitorService background Task.Delay). These continuations do not touch the UI (monitors emit via events).
• .ToList() -> .ToArray() on three methods whose return type is IReadOnlyList (ActivityLogService.GetRecent, LargeFileScanner.Scan, DnsService.CaptureCurrentServersAsync) — array satisfies the interface without keeping a List's spare capacity.

Audit false-positives deliberately NOT applied (verified against real code)

• ConfigureAwait on the expression-bodied => Task.Run(...) services: they RETURN the task without awaiting; all 13 such services use this exact uniform pattern, so adding it would force them async for zero benefit.
• new List(2) -> []: the capacity hint is intentional; left as-is.
• ServiceManagerService CancellationToken: WaitForStatus already has a 30s timeout and no caller has a token to pass.

Verification

• dotnet build (app + tests): 0 warnings, 0 errors.