feat(posts): add bookmark component

[stickerdaniel]

Hey, I know you want to keep this project minimal, but this is a great addition for users who plan to publish long posts. You can set the bookmark button behavior in the .env settings to enable it / enable it only on mobile. It provides a convenient way to continue reading / editing where the user left off by storing the scroll position in the browser's local storage. Looking forward to your feedback!

Changes

• Added new Bookmark.svelte component with scroll tracking logic
• Integrated bookmark into post pages
• Added configuration option to .env.example