fix: decouple timeline from workspace layout

The regression recurred because a floating timeline height was still exported as shared layout space and its z-index exceeded artifacts and notices. Reserve zero layout height and define the timeline below artifact and notification overlays.

src/features/workbench/workspace/workbench-main-frame.tsx

@@ -95,7 +95,10 @@ export function WorkbenchMainFrame({
             id="artifact-workspace"
             aria-label={`${activeArtifact.title}工作区`}
             aria-hidden={!showArtifact}
-            className={cn("relative min-h-0 min-w-0 overflow-hidden", !showArtifact && "hidden")}
+            className={cn(
+              "relative z-30 min-h-0 min-w-0 overflow-hidden",
+              !showArtifact && "hidden"
+            )}
           >
             <AnalysisArtifactWorkspace
               artifact={activeArtifact}
@@ -119,7 +122,7 @@ export function WorkbenchMainFrame({
       </div>
 
       <motion.div
-        className="pointer-events-auto absolute left-3 z-40 mx-auto min-h-0"
+        className="pointer-events-auto absolute left-3 z-20 mx-auto min-h-0"
         initial={false}
         animate={{
           bottom: timelineBottom,