fix(web): end first-run onboarding on any usable provider

The step and the Models page both asked one question of a join that
describes every provider: is deepseek-official's credential stored? A user
who configured some other route was taken over on every blank session, and
the DeepSeek setup card opened over them on every visit to Models with a
Cancel that could not close it — while clearing the add card's draft,
because it shared the row-editor close handler.

providerUsable(row) now answers what both surfaces need: the route is
registered and whatever credential its profile names is stored. Readiness
(renamed onboardingReadiness) ends on any usable row, needsSetup takes the
same fact, and each card kind owns its own close handler.

Fixes #2325

tsconfig.host.json

@@ -29,6 +29,7 @@
     "apps/web/tests/settings-chrome.e2e.ts",
     "apps/web/tests/models-settings.e2e.ts",
     "apps/web/tests/onboarding-deepseek-config.e2e.ts",
+    "apps/web/tests/onboarding-usable-provider.e2e.ts",
     "apps/web/tests/remote-welcome.e2e.ts",
     "apps/web/tests/workspace-management.e2e.ts",
     "apps/web/tests/replay-round-trip.e2e.ts",