fix: platform architecture picker visibility

[MattKiazyk]

Summary

• Determine available platform architectures before filtering runtimes
• Keep the architecture dropdown visible when both Universal and Apple Silicon variants are supported
• Hide the dropdown when the selected Xcode build supports only one architecture variant
• Automatically display the sole supported variant regardless of the previously saved selection
• Prevent unsupported architecture options from producing an empty platform list

Testing

• Ran the macOS test suite
• 35 tests passed with 0 failures