fix: support command+args style $EDITOR env

[Yukaii]

Summary

• Parse $EDITOR using shell-style splitting so command+args work (for example kks edit and quoted arguments)
• Validate empty/invalid $EDITOR values with a clear flash error
• Execute editor as binary plus args, appending the selected file path as the final argument

Testing

• go test ./ui ./config ./theme ./tree ./icons

[vercel]

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