🐛 Fix duplicate file extension when saving on macOS

[NielsBillen]

Problem

On macOS the file extension is added twice when saving a file (e.g. report.pdf.pdf). The issue is not present on Windows.

Root cause

NSSavePanel automatically appends the extension from allowedFileTypes to the entered file name. The save dialog also baked the extension into nameFieldStringValue via buildFileSaverSuggestedName, so the panel appended it a second time.

Windows is unaffected because its native IFileSaveDialog.SetDefaultExtension only appends an extension when the file name has none — it sets a bare name plus a separate default extension.

Fix

Mirror the Windows approach on both macOS code paths:

• macosMain/.../FileKit.macos.kt (native macOS target)
• jvmMain/.../platform/mac/MacOSFilePicker.kt (JVM / Desktop)

Set the bare suggestedName as the name field and let allowedFileTypes supply the extension. A new shared buildFileSaverAllowedFileTypes helper orders the file types with the default extension first, so NSSavePanel appends the default one.

Tests

Added unit tests for buildFileSaverAllowedFileTypes in FileSaverNameTest (commonTest). :filekit-dialogs:jvmTest passes locally.

Note: macosMain was not compiled locally (no macOS host available); the change is symmetric to the JVM path. A macOS CI run would be worth confirming.