"Add marketplace" stuck on "already added" after marketplace was removed

Summary:
After removing a plugin marketplace, re-adding the same source via the "Add marketplace" dialog fails with "This marketplace is already added," even though it no longer appears anywhere in the marketplace/plugin list. There is no way to re-add it through the UI.

Steps to reproduce:

1. Add marketplace FMTrainingTV-AI/rcc-fm (GitHub owner/repo shorthand).
2. Remove it via the app's remove/uninstall action.
3. Try to add it again with the same value, FMTrainingTV-AI/rcc-fm.

Expected: Marketplace is re-added successfully (or a "Sync" button that actually re-fetches, since none exists to sync at this point).

Actual: Dialog shows "This marketplace is already added," with only a "Sync" button offered — but there is nothing to sync since it was removed. No entry for it is visible in the marketplace/plugin list elsewhere in the app.

Also tried:

• Full git URL form (https://github.com/FMTrainingTV-AI/rcc-fm.git) — same "already added" error.
• Local filesystem path to a manually-downloaded/extracted copy of the repo — rejected outright with "Marketplace source format is invalid. Enter a GitHub owner/repo or a full git URL." (local paths are not accepted by this dialog at all)
• Fully quit and relaunched the app — no change, so this is not a client-side cache issue; the stuck state appears to be server-side.

Environment: Claude Code desktop app.