Support empty comments as highlights

[kylemcd]

What changed

• The Allow empty comments setting defaults to off.
• A blank field saves an empty comment for prose, inline code, and fenced code selections.
• Each empty comment shows an editable Empty card in the margin and sidebar.
• Add comment adds text to an exact empty comment or deletes it when the field stays blank.
• Reading view preserves mapping for repeated inline code, raw HTML, and boundary whitespace.
• A blank field closes without a change when the setting is off.

Why

• Users can mark important text before they have a comment to add.
• The Empty card keeps the comment visible and lets users edit it directly.
• Closes [Request] Highlight text without attaching a comment #52.

Notes

• No dedicated highlight command exists.
• Select Empty to add the first comment text.
• You can also select all highlighted text and run Add comment again.
• Checks: npm run check with 137 tests, npm run build, and the README STE lint.