[UX] Plugin market "Update / repair" action always fails #58

Summary

The plugin browser offers an Update / repair install action when an entry is installed and updateAvailable, but the installer hard-refuses every entry already present in the profile manifest. Every click on the advertised update path fails with "already or partially installed; uninstall it before reinstalling" — a guaranteed dead end that demands a manual uninstall plus two restarts.

Root cause

• packages/mayfly/src/interaction/plugin-commands.ts:312 (detail panel) and :370 (browse list) offer the action when updateAvailable; runOperation explicitly permits it (plugin-commands.ts:398-401).
• installEntry then rejects any entry whose rows are already in the profile manifest: packages/mayfly/src/interaction/plugin-market/installer.ts:303-306.

So the UI advertises an operation the backend refuses by construction. There is no upgrade path that replaces existing manifest rows.

Acceptance criteria

• "Update / repair" either performs a real upgrade (replace existing rows transactionally, keeping the installer's snapshot/rollback discipline) or is not offered for installed entries.
• The failure message, when reachable, no longer suggests the advertised-but-broken path.

Audit reference

docs/audits/2026-09-28-ui-ux-audit-flows-and-localization.md § UX-01 (baseline 27bc188, 2026-09-28)