fix(plugins): fail the delete action when no row was removed

[deanrie]

Summary

deletePluginAction runs .delete().eq("id", id).eq("owner_id", userId) through the user-scoped Supabase client and only checks error. PostgREST does not treat a delete that matches zero rows as an error, so when the row survives (RLS refusing the delete, or any other reason the filter matches nothing), the action still returns success, the UI shows "Plugin deleted." and navigates home, and the listing stays live with its slug and repository URL. That is the sequence in #413, #429 and #438, and #429 already points at this code.

This adds .select("id") to the delete and throws an ActionError when nothing came back, so the owner sees a real error toast instead of a false success. It does not change which rows the policy allows to be deleted; the stuck listings in those issues still need the backend cleanup, this just stops the UI from lying about it.

togglePluginListingAction already has the equivalent guard through .select("slug").single(), so unpublish is not touched.

Fixes #413, fixes #429, fixes #438.

Verification

• bun x biome ci . and bun run typecheck (apps/cursor) pass, same commands as ci.yml.
• The error message surfaces through handleServerError (ActionError is passed through) and plugin-owner-menu.tsx already renders error.serverError in the onError toast.

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[deanrie]

Superseded by #449, which carries this commit together with the other plugin-page fix as one reviewable PR.