fix: remove all unnecessary using directives (IDE0005)

- Fixed 14 files with IDE0005 errors
- Removed unused System.Threading, System.Runtime.InteropServices, etc.
- Verified build passes with -warnaserror:IDE0005
- All GitHub Actions builds should now succeed

Author: biancode

src/Commands/CommandWithRetry.cs

@@ -1,6 +1,5 @@
 using System;
 using System.IO;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace SourceGit.Commands

src/Commands/GitFlowFinish.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.Threading.Tasks;
 
 namespace SourceGit.Commands

src/Commands/Optimization/BatchQueryExecutor.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace SourceGit.Commands.Optimization

src/Commands/QueryBranchesOptimized.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using SourceGit.Commands.Optimization;
 

src/Models/CredentialManager.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
-using System.Runtime.InteropServices;
 using System.Text;
 
 namespace SourceGit.Models

src/Models/PerformanceMonitor.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Threading;
 
 namespace SourceGit.Models
 {

src/Models/RefreshOptions.cs

@@ -1,4 +1,3 @@
-using System;
 
 namespace SourceGit.Models
 {

src/Models/User.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;

src/ViewModels/Repository.Search.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
-using Avalonia.Threading;
 
 namespace SourceGit.ViewModels
 {

src/ViewModels/Repository.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
-using System.Text;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;