feat(ui): storybook 10 over the real pages + screenshot harness

Preview imports the mock transport statically and wraps every story in
a fresh RegistryProvider seeded with mock mode + the story's scenario
parameter (dark default, light via toolbar). One story file per page
(Default + FirstRun/Syncing/Errors where meaningful) plus the whole
shell. tools/screenshots.mjs mirrors the console's harness: headless
Chromium over storybook-static, every Pages/* + Shell/* story.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/App.stories.tsx

@@ -0,0 +1,14 @@
+import type { Meta, StoryObj } from "@storybook/react-vite";
+import { App } from "./App";
+
+const meta = {
+	title: "Shell/App",
+	component: App,
+	parameters: { layout: "fullscreen" },
+} satisfies Meta<typeof App>;
+
+export default meta;
+type Story = StoryObj<typeof meta>;
+
+export const Default: Story = {};
+export const FirstRun: Story = { parameters: { scenario: "firstRun" } };