fix(ui): align the right rail to where the left rail settles

The right rail took its top from the left stack's bounding rect while the
left stack's `top` was still transitioning (150 ms): the left layout sets
its target and calls onAligned() synchronously, the right pass runs a
frame later mid-transition, and nothing re-ran it when the transition
ended. The rail then sat ~32 px off and jumped on the next unrelated
relayout - under the pointer in CI's panel gates, which is why a header
press missed on every run.

settledLeftRailTop() reads the end value of a running `top` transition
(falling back to dataset.safeTopPct, then the rect). A settled stack is
read from its rect as before, which keeps Cyber's CSS top override (and
cockpit's transition: none) exactly as they were. The Cyber test's
pinned pattern follows the new helper.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/cyberTheme.test.mjs

@@ -712,7 +712,7 @@ test('the Display selector, voice schema and final stylesheet expose Cyber', ()
   );
   assert.match(
     rightRail,
-    /const leftStackTop = leftStack\?\.getBoundingClientRect\(\)\.top;[\s\S]*?const alignedTop = Number\.isFinite\(leftStackTop\)[\s\S]*?baseTop: alignedTop,/,
+    /const leftStackTop = settledLeftRailTop\([\s\S]*?const alignedTop = Number\.isFinite\(leftStackTop\)[\s\S]*?baseTop: alignedTop,/,
   );
   assert.match(
     cyberStyles,