Cover style "halves": left and right half blocks only

Twice the horizontal detail of "cells", for terminals such as macOS
Terminal whose upper and lower half blocks do not meet at the middle of
the cell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/settings.go

@@ -26,7 +26,7 @@ type settingsValues struct {
 	notify, bell                                      bool
 }
 
-var coverStyles = []string{"auto", "kitty", "sextant", "quadrant", "cells"}
+var coverStyles = []string{"auto", "kitty", "sextant", "quadrant", "halves", "cells"}
 
 // settingsFrom reads the settings in use.
 func settingsFrom(c *config.Config) settingsValues {