Fix white screen caused by theme defaulting to 'light'

The app is a dark-mode design but all theme defaults were 'light'.
Once the theme setting was wired to data-theme, it applied light mode
overrides that turned the app background white while panels stayed dark.

- Changed all server-side theme defaults from 'light' to 'dark'
- Added data-theme="dark" to index.html so the correct theme applies
  before React loads (eliminates any white flash on first render)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/public/index.html

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en">
+<html lang="en" data-theme="dark">
   <head>
     <meta charset="utf-8" />
     <link rel="icon" type="image/png" href="%PUBLIC_URL%/app-icon.png" />