Add active line highlight setting

[jsgrrchg]

Summary

Adds a new per-vault Editor setting for highlighting the active CodeMirror line.

Changes

• Adds editorActiveLineHighlight to desktop settings with a default of true.
• Wires CodeMirror highlightActiveLine() and highlightActiveLineGutter() through a reconfigurable compartment.
• Adds the Settings > Editor > Formatting toggle.
• Applies the preference to both note editors and text-file editors.
• Documents the setting scope/default in docs/settings-scope.md.

Validation

• npm --prefix apps/desktop run build
• npm --prefix apps/desktop run lint