Fix file dialog on modern macOS (13+)

[snegnik]

Summary

• On modern macOS (13–15), clicking "Open mesh" does nothing — the file dialog never appears
• Root cause: NanoGUI's file_dialog() calls NSOpenPanel.runModal which returns immediately with NSModalResponseCancel when invoked within GLFW's event loop
• Fix: Replace with an osascript-based native file dialog that runs in a separate process, bypassing the GLFW event loop conflict
• Guarded with #ifdef __APPLE__ — Linux and Windows builds are unaffected

Changed files

File	Change
src/native_file_dialog.h	New header declaring native_file_dialog()
src/native_file_dialog.cpp	Implementation using osascript / AppleScript choose file
src/viewer.cpp	Route all file_dialog calls through im_file_dialog macro
CMakeLists.txt	Include new source file on Apple builds

Test plan

• Build on macOS (Apple Silicon, macOS 15)
• "Open mesh" button opens native macOS file picker
• Selecting an OBJ/PLY file loads the mesh successfully
• No crashes
• Verify Linux/Windows builds are unaffected

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