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/build/index.html

@@ -1 +1 @@
-<!doctype html><html lang="en"><head><meta charset="utf-8"/><link rel="icon" type="image/png" href="/app-icon.png"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/app-icon.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.1e7808e0.js"></script><link href="/static/css/main.67d2dbc4.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>
+<!doctype html><html lang="en" data-theme="dark"><head><meta charset="utf-8"/><link rel="icon" type="image/png" href="/app-icon.png"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/app-icon.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.1e7808e0.js"></script><link href="/static/css/main.67d2dbc4.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>