A persistent left-rail toolbar (APG toolbar + roving tabindex) toggles the
library / editor / preview panes; a hidden pane frees its space and the rest
redistribute proportionally to their remembered widths. Visibility persists to
astrolabe:ux-prefs (separate key from widths, so a hidden pane keeps its width).
Adds the positional pane-* glyph sub-family to Icon and a Datasets shortcut
button in the strip.
