fix: stage flix.jar download to avoid Windows file locking

[magnus-madsen]

Summary

Fixes #534.

• On Windows the JVM holds a file lock on flix.jar, so replacing it mid-session fails with a file-locking error.
• Updates are now downloaded to flix.jar.new instead, and the user is prompted to reload the window.
• On the next startup (before the JVM launches), flix.jar.new is renamed to flix.jar.
• The changelog display is deferred to after the rename so it is not lost on reload.

Test plan

• First install: Remove globalStoragePath, activate extension — should download directly to flix.jar (no .new staging)
• Mid-session update: With a running session and a newer compiler available, accept the download — should create flix.jar.new and show "Reload Now" prompt (no "Starting Flix" spinner)
• Reload applies update: After downloading .new, click "Reload Now" — .new should be renamed to .jar, changelog shown, extension starts with new version
• Dismiss reload: Download update, dismiss the reload prompt — old jar continues working; update applied on next VS Code restart
• Windows: Verify the above on Windows specifically (the platform where the bug occurs)

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