Fix notification panel readability — dark theme styling

Replace hard-coded white/light colors with dark theme palette matching the rest
of the app. Key changes: dark navy background (#1e293b/#0f172a), high-contrast
text (#f1f5f9 titles, #94a3b8 body), teal accent (#14b8a6) for active filter and
unread left-border, red (#f87171) clear button, dark scrollbar, subtle hover
states on items and action buttons.

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.a387bcf0.js"></script><link href="/static/css/main.40bbff16.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"><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.a387bcf0.js"></script><link href="/static/css/main.5afb44a9.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>