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/Input.stories.tsx

@@ -0,0 +1,40 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { Input } from '../../src/ui/Input/Input';
+
+const meta: Meta<typeof Input> = {
+  title: 'UI/Input',
+  component: Input,
+  parameters: { layout: 'centered' },
+  tags: ['autodocs'],
+  decorators: [
+    (Story) => (
+      <div style={{ width: 320 }}>
+        <Story />
+      </div>
+    ),
+  ],
+};
+export default meta;
+type Story = StoryObj<typeof Input>;
+
+export const Default: Story = {
+  args: { label: 'Label', placeholder: 'Placeholder' },
+};
+export const WithHint: Story = {
+  args: {
+    label: 'Environment name',
+    placeholder: 'e.g. staging',
+    hint: 'Used to identify this environment in scenarios.',
+  },
+};
+export const WithError: Story = {
+  args: {
+    label: 'URL',
+    placeholder: 'https://...',
+    defaultValue: 'not a url',
+    error: 'Must be a valid URL.',
+  },
+};
+export const Disabled: Story = {
+  args: { label: 'Label', value: 'Some value', disabled: true, readOnly: true },
+};