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/hooks/useTheme.ts

@@ -0,0 +1,26 @@
+import { useState, useEffect } from 'react';
+
+type Theme = 'light' | 'dark';
+
+const STORAGE_KEY = 'qa-bot-theme';
+
+function getInitialTheme(): Theme {
+  const stored = localStorage.getItem(STORAGE_KEY);
+  if (stored === 'light' || stored === 'dark') return stored;
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}
+
+export function useTheme() {
+  const [theme, setTheme] = useState<Theme>(getInitialTheme);
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+    localStorage.setItem(STORAGE_KEY, theme);
+  }, [theme]);
+
+  function toggleTheme() {
+    setTheme(t => (t === 'light' ? 'dark' : 'light'));
+  }
+
+  return { theme, toggleTheme } as const;
+}