Tool page UI improvements

We've improved the usability and clarity of the tool page.

View mode switcher
The Edit / Preview / Split options are now a single segment control with icons, making the current selection easier to see.

Toolbar layout
The Reset button is grouped on the right with the Layout settings button, so the view mode controls on the left are clearly separated from actions on the right.

We've also improved line break handling in Markdown and added GitHub-style syntax highlighting for code blocks. If you have any feedback, please let us know.