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.

.storybook/preview.js

@@ -1,10 +1,43 @@
 import "../app/globals.css";
 import "./fonts.css";
+import { MINIMAL_VIEWPORTS } from "storybook/viewport";
+import { AuthModalProvider } from "../app/contexts/AuthModalContext";
 import { MessagesProvider } from "../app/contexts/MessagesContext";
+import { CreateFlowProvider } from "../app/(app)/create/context/CreateFlowContext";
 import messages from "../messages/en/index";
 
+/** Figma / Tailwind breakpoints from `app/tailwind.css`. */
+const dsViewports = {
+  sm: {
+    name: "sm 430",
+    styles: { width: "430px", height: "932px" },
+  },
+  md: {
+    name: "md 640",
+    styles: { width: "640px", height: "1024px" },
+  },
+  lg: {
+    name: "lg 1024",
+    styles: { width: "1024px", height: "768px" },
+  },
+  xl: {
+    name: "xl 1440",
+    styles: { width: "1440px", height: "900px" },
+  },
+  // Aliases used by existing page stories
+  mobile1: {
+    name: "sm 430",
+    styles: { width: "430px", height: "932px" },
+  },
+  desktop: {
+    name: "xl 1440",
+    styles: { width: "1440px", height: "900px" },
+  },
+};
+
 /** @type { import('@storybook/react').Preview } */
 const preview = {
+  tags: ["autodocs"],
   parameters: {
     controls: {
       matchers: {
@@ -12,13 +45,32 @@ const preview = {
         date: /Date$/i,
       },
     },
+    backgrounds: {
+      options: {
+        dark: { name: "Dark", value: "#000000" },
+        light: { name: "Light", value: "#ffffff" },
+      },
+    },
+    viewport: {
+      options: {
+        ...MINIMAL_VIEWPORTS,
+        ...dsViewports,
+      },
+    },
+  },
+  initialGlobals: {
+    backgrounds: { value: "dark" },
   },
   decorators: [
     (Story) => (
       <MessagesProvider messages={messages}>
-        <div className="font-inter">
-          <Story />
-        </div>
+        <AuthModalProvider>
+          <CreateFlowProvider>
+            <div className="font-inter">
+              <Story />
+            </div>
+          </CreateFlowProvider>
+        </AuthModalProvider>
       </MessagesProvider>
     ),
   ],