Add a native first-run start page instead of loading example.com

[culpen90]

Why

Quartz currently uses https://www.example.com as its built-in home URL in Sources/Quartz/QuartzApp.swift. That works as a placeholder, but it makes first launch feel unfinished and does not help new users discover the browser's features.

Where to start

• Sources/Quartz/QuartzApp.swift
• README.md if the new behavior should be documented

Suggested approach

Replace the placeholder home page with a simple native/local Quartz start page. A good first version could show a search/address entry, a short list of built-in features, and links to extension install docs or the repository.

Keep the implementation small and native to the existing AppKit/WebKit flow. A local bundled HTML page or generated data:/file URL is fine if it keeps the app easy to run with swift run Quartz.

Acceptance criteria

• First launch no longer opens https://www.example.com.
• The Home button returns to the new Quartz start page.
• The address field remains usable for normal URLs and searches.
• Session restore still wins when a valid previous browsing URL exists.
• The app still builds with swift build.

Testing

• Run swift build.
• Run swift run Quartz from a clean/defaults-cleared state and confirm the first page is the new Quartz start page.
• Navigate elsewhere, use the Home button, and confirm it returns to the start page.