feat: add theme provider and toggle functionality to support light/dark

[kerwin2046]

Summary

This PR improves the starter kit's dark mode behavior by introducing a global ThemeProvider and
making the welcome screen's dark mode toggle use it.

• Add a ThemeProvider with a useTheme hook to manage the app theme (light / dark) in a single
  place.
• Initialize the theme from localStorage (when available) or from the system prefers-color-scheme
  media query.
• Persist the selected theme in localStorage so it survives app restarts.
• Apply the dark class on document.documentElement to integrate with Tailwind's dark mode
  styling.
• Refactor WelcomeKit's dark mode toggle to use useTheme instead of mutating
  document.documentElement directly.

Implementation details

• New file app/components/theme/ThemeProvider.tsx:
  • Exposes ThemeProvider and useTheme.
  • Resolves the initial theme in this order: localStorage → system preference → default light.
  • Listens to prefers-color-scheme: dark changes when the user has not explicitly chosen a theme.
  • Keeps document.documentElement in sync by adding/removing the dark class.
• app/renderer.tsx:
  • Wraps <App /> with ThemeProvider inside WindowContextProvider.
• app/components/welcome/WelcomeKit.tsx:
  • Replaces direct DOM manipulation with useTheme.
  • The toggle label now reflects the current theme (Dark Mode / Light Mode).

Testing

• Run npm install and npm run dev.
• Open the app and:
  • Toggle dark mode using the badge in the welcome screen and confirm the UI updates.
  • Reload the app and confirm the chosen theme is preserved.
  • (Optional) Change the OS theme and restart the app without manually toggling, to see it pick up
    the system preference when no explicit choice is stored.