Initial commit: .NET MAUI Linux Platform

Complete Linux platform implementation for .NET MAUI with:

- 35+ Skia-rendered controls (Button, Label, Entry, CarouselView, etc.)
- Platform services (Clipboard, FilePicker, Notifications, DragDrop, etc.)
- Accessibility support (AT-SPI2, High Contrast)
- HiDPI and Input Method support
- 216 unit tests
- CI/CD workflows
- Project templates
- Documentation

🤖 Generated with Claude Code

Services/FolderPickerService.cs

@@ -0,0 +1,129 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+
+namespace Microsoft.Maui.Platform.Linux.Services;
+
+/// <summary>
+/// Linux folder picker utility using zenity or kdialog.
+/// This is a standalone service as MAUI core does not define IFolderPicker.
+/// </summary>
+public class FolderPickerService
+{
+    public async Task<string?> PickFolderAsync(string? initialDirectory = null, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            // Try zenity first (GNOME)
+            var result = await TryZenityFolderPicker(initialDirectory, cancellationToken);
+            if (result != null)
+            {
+                return result;
+            }
+
+            // Fall back to kdialog (KDE)
+            result = await TryKdialogFolderPicker(initialDirectory, cancellationToken);
+            if (result != null)
+            {
+                return result;
+            }
+
+            return null;
+        }
+        catch (OperationCanceledException)
+        {
+            return null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private async Task<string?> TryZenityFolderPicker(string? initialDirectory, CancellationToken cancellationToken)
+    {
+        try
+        {
+            var args = "--file-selection --directory";
+            if (!string.IsNullOrEmpty(initialDirectory) && Directory.Exists(initialDirectory))
+            {
+                args += $" --filename=\"{initialDirectory}/\"";
+            }
+
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "zenity",
+                Arguments = args,
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return null;
+
+            var output = await process.StandardOutput.ReadToEndAsync(cancellationToken);
+            await process.WaitForExitAsync(cancellationToken);
+
+            if (process.ExitCode == 0 && !string.IsNullOrWhiteSpace(output))
+            {
+                var path = output.Trim();
+                if (Directory.Exists(path))
+                {
+                    return path;
+                }
+            }
+
+            return null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private async Task<string?> TryKdialogFolderPicker(string? initialDirectory, CancellationToken cancellationToken)
+    {
+        try
+        {
+            var args = "--getexistingdirectory";
+            if (!string.IsNullOrEmpty(initialDirectory) && Directory.Exists(initialDirectory))
+            {
+                args += $" \"{initialDirectory}\"";
+            }
+
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "kdialog",
+                Arguments = args,
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return null;
+
+            var output = await process.StandardOutput.ReadToEndAsync(cancellationToken);
+            await process.WaitForExitAsync(cancellationToken);
+
+            if (process.ExitCode == 0 && !string.IsNullOrWhiteSpace(output))
+            {
+                var path = output.Trim();
+                if (Directory.Exists(path))
+                {
+                    return path;
+                }
+            }
+
+            return null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+}