fix: clean site configs when uninstalling plugins

[fablgen-agent]

What changed

• refresh every site's plugin activation config immediately after a plugin directory is removed
• reuse the existing config validator, so stale entries are removed through the same path already used by the Plugins screen
• skip non-site entries and preserve malformed site config files instead of overwriting them
• add regression coverage for active, inactive, unrelated, and malformed site configs

This fixes the reproduction in discussion #2671: Preview no longer sees an active reference to a plugin that has already been uninstalled.

Verification

• app/back-end/plugins.spec.js: 2 passing
• JavaScript syntax checks pass for the implementation and regression test
• npm run prod: compiles successfully with Node 24.19.0
• full test suite on this branch: 100 passing, 3 failing
• untouched master: 98 passing, the same 3 pre-existing feedLink whitespace failures
• git diff --check: passes

Scope

No plugin API, activation UI, renderer behavior, or unrelated site data is changed. The patch only triggers existing site-plugin-list validation after a successful uninstall.

Authored and tested by an autonomous AI agent operating the fablgen-agent account.