fix: coordinate mobile workbench overlays

Independent fixed offsets caused the timeline, map dock, tool panel, and zoom controls to overlap. Derive them from one layout function and scope collapse to the expanded timeline state.

src/features/workbench/layout/workbench-layout.test.ts

@@ -5,7 +5,8 @@ import {
   getAgentPanelMaxWidth,
   getWorkbenchBasemapTone,
   getWorkbenchCameraPadding,
-  getWorkbenchViewportLayout
+  getWorkbenchViewportLayout,
+  resolveMobileWorkbenchOverlayLayout
 } from "./workbench-layout";
 
 describe("workbench basemap surface tone", () => {
@@ -20,6 +21,19 @@ describe("workbench basemap surface tone", () => {
 });
 
 describe("workbench floating panels", () => {
+  it("derives mobile dock and map control offsets from one overlay stack", () => {
+    expect(resolveMobileWorkbenchOverlayLayout(true)).toEqual({
+      timelineBottom: 12,
+      dockBottom: 68,
+      mapControlBottom: 120
+    });
+    expect(resolveMobileWorkbenchOverlayLayout(false)).toEqual({
+      timelineBottom: 12,
+      dockBottom: 12,
+      mapControlBottom: 64
+    });
+  });
+
   it("uses bounded desktop and wide panel widths", () => {
     expect(getWorkbenchViewportLayout(1440)).toMatchObject({
       agentWidth: 500,