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/index.ts

@@ -19,5 +19,7 @@ export type { SidePanelProps } from './SidePanel/SidePanel';
 export { Breadcrumbs } from './Breadcrumbs/Breadcrumbs';
 export type { BreadcrumbsProps, BreadcrumbItem } from './Breadcrumbs/Breadcrumbs';
 
+export { ThemeSwitcher } from './ThemeSwitcher/ThemeSwitcher';
+
 export { Table } from './Table/Table';
 export type { TableProps, TableColumn } from './Table/Table';