feat(onboarding): hide 0-widget integrations and add selectable app grid

- Filter out integrations with no widgets from onboarding selection
- Replace single checkbox app toggle with a compact selectable grid
- Each Docker-discovered app is individually toggleable (all on by default)
- Track selected apps by container ID instead of a boolean flag

apps/nextjs/src/components/integration/integration-grid-shared.ts

@@ -74,6 +74,7 @@ export const buildSortedIntegrations = (
   integrationKinds
     .filter((kind) => {
       if (options.onboarding && hiddenFromOnboarding.has(kind)) return false;
+      if (options.onboarding && widgetsByIntegration[kind].length === 0) return false;
       if (!options.enableMockIntegration && kind === "mock") return false;
       return true;
     })