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/src/ui/Card/Card.stories.tsx

@@ -1,46 +0,0 @@
-import React from 'react';
-import type { Meta, StoryObj } from '@storybook/react-vite';
-import { Card } from './Card';
-import { Badge } from '../Badge/Badge';
-import { Button } from '../Button/Button';
-
-const meta: Meta<typeof Card> = {
-  title: 'UI/Card',
-  component: Card,
-  parameters: { layout: 'centered' },
-  tags: ['autodocs'],
-  decorators: [
-    (Story) => (
-      <div style={{ width: 360 }}>
-        <Story />
-      </div>
-    ),
-  ],
-  argTypes: {
-    padding: { control: 'select', options: ['sm', 'md', 'lg'] },
-  },
-};
-export default meta;
-type Story = StoryObj<typeof Card>;
-
-export const Default: Story = {
-  args: { children: 'Card content goes here.', padding: 'md' },
-};
-
-export const ScenarioCard: Story = {
-  render: () => (
-    <Card>
-      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'flex-start', marginBottom: 8 }}>
-        <span style={{ fontWeight: 600, fontSize: 14 }}>Login flow</span>
-        <Badge variant="success" dot>Passed</Badge>
-      </div>
-      <p style={{ margin: '0 0 16px', fontSize: 13, color: 'var(--color-text-muted)' }}>
-        Last run 2 minutes ago · 4 steps
-      </p>
-      <div style={{ display: 'flex', gap: 8 }}>
-        <Button size="sm" variant="primary">Run</Button>
-        <Button size="sm" variant="ghost">Edit</Button>
-      </div>
-    </Card>
-  ),
-};