Remove redundant System using in ServerConnection

[erikdarlingdata]

Summary

• Drop using System; from ServerConnection.cs — already covered by .NET 8 implicit global usings.
• Clears LSP warnings CS8933 (duplicate of global using) and CS8019 (unnecessary using).

Test plan

• dotnet build is clean
• No new diagnostics on the file

🤖 Generated with Claude Code

[erikdarlingdata]

Review

What it does: Removes using System; from src/PlanViewer.Core/Models/ServerConnection.cs:1. Single-line deletion.

Verified:

• PlanViewer.Core.csproj:5 has <ImplicitUsings>enable</ImplicitUsings> on net8.0. The .NET SDK implicit global usings include System, so Guid (line 9), DateTime (lines 14–15), and InvalidOperationException (line 60) still resolve.
• Base branch is dev — correct.
• No Avalonia, XAML, or PlanAnalyzer changes; nothing to mirror to PerformanceMonitor Dashboard/Lite.
• No src/ logic change, so no test gap in tests/PlanViewer.Core.Tests/.

Needs attention: None. Confirm dotnet build is clean as the PR's test plan calls out.

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