feat(client): add ThemeSwitcher and relocate stories to .storybook/

- add lucide-react for Sun/Moon icons
- add useTheme hook persisting choice to localStorage with prefers-color-scheme fallback
- add ThemeSwitcher button component wired into SidePanel header
- add blocking inline script to index.html to prevent flash-of-wrong-theme
- move all *.stories.tsx from src/ into .storybook/stories/ and update imports
- update main.ts stories glob to .storybook/stories/**

client/.storybook/stories/Breadcrumbs.stories.tsx

@@ -0,0 +1,48 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { Breadcrumbs } from '../../src/ui/Breadcrumbs/Breadcrumbs';
+
+const meta: Meta<typeof Breadcrumbs> = {
+  title: 'UI/Breadcrumbs',
+  component: Breadcrumbs,
+  parameters: { layout: 'centered' },
+  tags: ['autodocs'],
+};
+export default meta;
+type Story = StoryObj<typeof Breadcrumbs>;
+
+export const Default: Story = {
+  args: {
+    items: [
+      { label: 'Scenarios', href: '#' },
+      { label: 'Login flow' },
+    ],
+  },
+};
+
+export const Deep: Story = {
+  args: {
+    items: [
+      { label: 'Home', href: '#' },
+      { label: 'Scenarios', href: '#' },
+      { label: 'Login flow', href: '#' },
+      { label: 'Step 3' },
+    ],
+  },
+};
+
+export const SingleItem: Story = {
+  args: {
+    items: [{ label: 'Dashboard' }],
+  },
+};
+
+export const CustomSeparator: Story = {
+  args: {
+    separator: '/',
+    items: [
+      { label: 'Environments', href: '#' },
+      { label: 'Staging', href: '#' },
+      { label: 'Config' },
+    ],
+  },
+};