Make the world timeline a scrubber under the dock

The timeline shows whenever a layer with time to show is on, live or not.
Its track runs over the time those layers cover, reaching to now, with a
tick for each launch or frame and a handle to drag; arrows step between the
ticks, and a scrub to now goes live. The bar sits along the bottom of the
screen, and the dock and everything anchored above it rise by its height;
clean view, recording and the cockpit put it away with the dock.

src/ui/mobileShell.test.mjs

@@ -586,7 +586,7 @@ test('the phone shell keeps the required attribution visible', () => {
   };
   const lift = (block) =>
     Number(
-      /bottom: calc\(var\(--mobile-tab-space\) \+ (\d+)px\)/.exec(block)?.[1],
+      /bottom: calc\(var\(--mobile-tab-space\) \+ (\d+)px \+ var\(--world-timeline-lift, 0px\)\)/.exec(block)?.[1],
     );
   const z = (block) => Number(/z-index: (\d+);/.exec(block)?.[1]);
   const idleCredit = rule(