feat: add configurable color scheme and dark branding

[ZiYan416]

Summary

• add a configurable status-page color scheme;
• support auto, light, and dark modes;
• add optional dark-mode logo and favicon assets;
• preserve the existing system-following behavior by default;
• apply the same color-scheme configuration to ColorSchemeScript and MantineProvider.

Configuration

const pageConfig: PageConfig = {
  colorScheme: 'auto',
  logo: '/logo-light.svg',
  darkLogo: '/logo-dark.svg',
  favicon: '/favicon-light.png',
  darkFavicon: '/favicon-dark.png',
}

auto follows the system color scheme.

Fallback behavior

• dark logo: darkLogo → logo → /logo.svg
• light logo: logo → /logo.svg
• dark favicon: darkFavicon → favicon → /favicon.png
• light favicon: favicon → /favicon.png

Existing configurations that do not use the new fields continue to behave exactly as before.

Scope

This PR intentionally does not add:

• an end-user theme toggle;
• localStorage theme preferences;
• new dependencies;
• custom theme colors;
• changes to monitoring, Worker, D1, proxy, or deployment logic.

Testing

• npm ci
• npm run lint
• npx tsc --noEmit
• npm run build
• default configuration follows the system theme
• forced light mode works while the system is dark
• forced dark mode works while the system is light
• dark logo and favicon are selected correctly
• missing dark assets fall back to the existing assets
• home page browser test
• incidents page
• embedded single-monitor browser test
• desktop and mobile layouts
• system theme changes are reflected while using auto

Closes #184

[ZiYan416]

I didn't configure darkFavicon during my initial testing, so the recorded GIF shows the fallback behavior using the standard favicon. I have now added the test for this, and as shown in the attached image, the feature works perfectly.

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