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/Button.stories.tsx

@@ -0,0 +1,26 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { Button } from '../../src/ui/Button/Button';
+
+const meta: Meta<typeof Button> = {
+  title: 'UI/Button',
+  component: Button,
+  parameters: { layout: 'centered' },
+  tags: ['autodocs'],
+  argTypes: {
+    variant: { control: 'select', options: ['primary', 'secondary', 'ghost', 'danger'] },
+    size: { control: 'select', options: ['sm', 'md', 'lg'] },
+    loading: { control: 'boolean' },
+    disabled: { control: 'boolean' },
+  },
+};
+export default meta;
+type Story = StoryObj<typeof Button>;
+
+export const Primary: Story = { args: { children: 'Button', variant: 'primary' } };
+export const Secondary: Story = { args: { children: 'Button', variant: 'secondary' } };
+export const Ghost: Story = { args: { children: 'Button', variant: 'ghost' } };
+export const Danger: Story = { args: { children: 'Delete', variant: 'danger' } };
+export const Small: Story = { args: { children: 'Small', size: 'sm' } };
+export const Large: Story = { args: { children: 'Large', size: 'lg' } };
+export const Loading: Story = { args: { children: 'Saving…', loading: true } };
+export const Disabled: Story = { args: { children: 'Button', disabled: true } };