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/main.ts

@@ -13,8 +13,8 @@ function getAbsolutePath(value: string) {
 }
 const config: StorybookConfig = {
   "stories": [
-    "../src/**/*.mdx",
-    "../src/**/*.stories.@(js|jsx|mjs|ts|tsx)"
+    "./stories/**/*.mdx",
+    "./stories/**/*.stories.@(js|jsx|mjs|ts|tsx)"
   ],
   "addons": [
     getAbsolutePath('@storybook/addon-vitest'),