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/ThemeSwitcher.stories.tsx

@@ -0,0 +1,13 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { ThemeSwitcher } from '../../src/ui/ThemeSwitcher/ThemeSwitcher';
+
+const meta: Meta<typeof ThemeSwitcher> = {
+  title: 'UI/ThemeSwitcher',
+  component: ThemeSwitcher,
+  parameters: { layout: 'centered' },
+  tags: ['autodocs'],
+};
+export default meta;
+type Story = StoryObj<typeof ThemeSwitcher>;
+
+export const Default: Story = {};