Quote CLI path in desktop launcher for Windows paths with spaces

[Copilot]

The desktop launcher fails on Windows when the project directory contains spaces (e.g., C:\Users\John Doe\...) because the unquoted path passed to spawn with shell: true is split by cmd.exe into multiple tokens.

Changes:

• Wrap mastraBinary path in quotes before passing to spawn to handle Windows paths with spaces
• Ensures desktop app can launch the agent for Windows users with default user directories

// Before: fails with paths containing spaces
const cli = spawn(`${mastraBinary} agent`, ...)

// After: handles spaces correctly
const cli = spawn(`"${mastraBinary}" agent`, ...)

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