fix(desktop): hold loading until pre-workspace overlay opens

[marovole]

Summary

• Fixes a startup white screen for authenticated users who still need a pre-workspace flow (onboarding, pending invitations, or create workspace).
• Root cause: DesktopShell rendered before the window overlay useEffect ran (and before listMyInvitations() resolved), leaving TabContent and WindowOverlay both null.
• Keeps the existing MulticaIcon loading screen up until the overlay is mounted.

Test plan

• Fresh account / onboarded_at null: app shows loading briefly, then onboarding or invitations overlay (no blank window).
• Onboarded user with zero workspaces: loading → create-workspace overlay.
• Onboarded user with workspace(s): normal dashboard, no extra loading delay.
• Logged-out cold start: login page unchanged.

Made with Cursor

[vercel]

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