Add pane show/hide toggle strip with persisted visibility (M6, §01A)

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.

src/app/components/PaneToggleStrip.module.css

@@ -0,0 +1,57 @@
+/*
+ * The pane toggle strip — a slim vertical rail at the left edge of the work area
+ * (spec §01A). Always present (never hidden), so it's the way back when panes are
+ * hidden. Visual look only; semantics live in PaneToggleStrip.tsx.
+ */
+.strip {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: var(--space-2);
+  flex: 0 0 auto;
+  padding: var(--space-3) var(--space-2);
+  border-right: var(--border-width) solid var(--border);
+  background: var(--layer-01);
+}
+
+.button {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 32px;
+  height: 32px;
+  padding: 0;
+  border: var(--border-width) solid transparent;
+  border-radius: var(--radius);
+  background: transparent;
+  /* Not-pressed (hidden pane) reads as muted; pressed (shown) lifts to full text. */
+  color: var(--text-secondary);
+  cursor: pointer;
+  transition:
+    background var(--dur-fast) var(--ease),
+    color var(--dur-fast) var(--ease);
+}
+
+.button:hover {
+  background: var(--layer-02);
+  color: var(--text);
+}
+
+.button:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: 2px;
+}
+
+/* Shown pane: the toggle is "on" — full-strength glyph on a filled chip. */
+.pressed {
+  color: var(--text);
+  background: var(--layer-02);
+  border-color: var(--border-strong);
+}
+
+.divider {
+  width: 20px;
+  height: var(--border-width);
+  margin: var(--space-1) 0;
+  background: var(--border);
+}