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/package.json

@@ -11,6 +11,7 @@
     "build-storybook": "storybook build"
   },
   "dependencies": {
+    "lucide-react": "^1.7.0",
     "react": "^19.1.0",
     "react-dom": "^19.1.0"
   },