layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USER-GUIDE.md

@@ -81,7 +81,7 @@ up to **4** plots, and the figure up to **8** in all.
 - **To split a row**, click the `+` at its left or right edge, or the `+` between two of its plots. The
   new plot is empty and the plots after it keep their traces. The `+` is dim at 4 plots in the row or 8
   in the figure.
-- **To resize**, drag the arrow between two plots left or right. The two neighbours trade width (each
+- **To resize**, drag the double-headed arrow between two plots left or right. The two neighbours trade width (each
   stays at least about 96 px wide); the sizes are saved with the session.
 - **To remove a plot**, click its `×`. In a split row it removes just that plot and its traces; in a row
   of one plot it removes the row.