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/App.module.css

@@ -7,6 +7,13 @@
   overflow: hidden;
 }
 
+.header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  width: 100%;
+}
+
 .brand {
   font-weight: 700;
   font-size: 1rem;