* fix(app): only offer Remove for providers defined in config.json
Plugin-registered providers (Antigravity, Muse Code) share source: "custom"
with config-defined ones, so Settings → Providers offered Remove on them.
The removeProviders patch only deletes config.json entries, so the action
succeeded silently while the plugin re-registered the provider.
Gate the Remove menu on the provider having an extends entry in config —
the actual definition custom providers live behind. Plugin providers keep
their enable/disable switch.
Fixes#6349
* Address review: named ownership check, E2E coverage for plugin providers
- Name the config-owned provider check instead of combining three
conditions inline in canRemove.
- Replace the JSDOM component tests with a Playwright case in the
existing provider-removal suite: a plugin-registered provider renders
without the Remove action and survives a removeProviders patch.
