Smarter list editing and polish

Improved

• Numbered list continuation — pressing Return inside a numbered list automatically inserts the next number, and subsequent items renumber when you add or remove lines mid-list.
• Heading shortcuts on empty lines — ⌥ ⌘ 1 → 6 now insert the heading prefix immediately on a blank line so you can start typing right away.
• Preview spacing — more breathing room between paragraphs and around lists, with balanced gaps on list-to-paragraph transitions.
• Sidebar's Add Location button is subtler by default, brightening on hover.

Fixed

• Heading syntax highlighting no longer bleeds into the line below when a heading line ends with only the prefix.