Sidebar: give the collapse arrow one home in the rail's top-right

On desktop the collapse/expand control swapped places between states: in
the brand row when expanded, but a nav row in the middle of the icon
column when collapsed — it read as the arrow jumping around.

- One button for both directions, anchored to the rail's top-right corner
  in both states (measured 23px from the rail's right edge either way;
  the padding accounts for the rest).
- Collapsed, the brand mark leads the icon column (it cannot share the
  brand row with the arrow: the 60px panel has ~40px of content width).
- The expanded state is unchanged (logo + name + DEV badge, arrow top
  right).

README: note in decision 22. SW -> freezmo-v41.

README.md

@@ -1054,7 +1054,9 @@ Things I chose while building, with the reasoning:
     rounded glass panel inset from the viewport edges, so the ice scene shows around it; deck cards
     are 24px-radius glass panels with a secondary Study CTA and quiet metadata. Headings are
     semibold with tighter tracking. Nested glass (chips, inputs) is more opaque but never adds a
-    second blur layer.
+    second blur layer. The collapse/expand arrow keeps one home — the rail's top-right corner in
+    both states (it used to become a nav row in the middle of the icon column when collapsed,
+    which read as the control jumping around); collapsed, the brand mark leads the icon column.
 23. **One fixed ice scene instead of per-page blobs.** The decorative background used to be
     rendered inside the content column, so on wide screens it was clipped into a visible hard edge.
     It now lives in a single `position: fixed` layer (`.ice-scene`, `z-index: -1`, pointer-events