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/Badge.stories.tsx

@@ -0,0 +1,25 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { Badge } from '../../src/ui/Badge/Badge';
+
+const meta: Meta<typeof Badge> = {
+  title: 'UI/Badge',
+  component: Badge,
+  parameters: { layout: 'centered' },
+  tags: ['autodocs'],
+  argTypes: {
+    variant: {
+      control: 'select',
+      options: ['success', 'error', 'warning', 'info', 'neutral', 'running'],
+    },
+    dot: { control: 'boolean' },
+  },
+};
+export default meta;
+type Story = StoryObj<typeof Badge>;
+
+export const Success: Story = { args: { children: 'Passed', variant: 'success', dot: true } };
+export const Error: Story = { args: { children: 'Failed', variant: 'error', dot: true } };
+export const Warning: Story = { args: { children: 'Warning', variant: 'warning' } };
+export const Info: Story = { args: { children: 'Info', variant: 'info' } };
+export const Neutral: Story = { args: { children: 'Pending', variant: 'neutral' } };
+export const Running: Story = { args: { children: 'Running', variant: 'running', dot: true } };