fix(app): only offer Remove for providers defined in config.json (#6568)

* 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.

.gitignore

@@ -93,3 +93,6 @@ app.json
 
 # Heap snapshots dumped by --heapsnapshot-near-heap-limit; keep them for debugging.
 *.heapsnapshot
+
+# Project-local Serena configuration and memories.
+.serena/