chore: catch up Storybook preview, coverage, and story hygiene

Stories were throwing without AuthModal/CreateFlow providers, autodocs was a no-op, and several DS/create-flow screens had no stories.

knip.json

@@ -25,7 +25,6 @@
     "@types/mdx",
     "eslint-config-next",
     "typescript-eslint",
-    "@storybook/nextjs-vite",
     "@eslint/js",
     "@next/eslint-plugin-next",
     "eslint-plugin-react",