fix(ui): replace hardcoded height calc with flex-1 to eliminate bottom blank area

All content panels used h-[calc(100vh-8rem)] which didn't match the
actual available space (CONTENT_TOP_OFFSET is 92px on Mac, 64px on
Win/Linux, not 128px), causing a visible gap at the bottom.

Replace with flex-1 min-h-0 to let panels fill the remaining space
after <main> and any sibling elements (e.g. OpenClaw health banner).

src/components/sessions/SessionManagerPage.tsx

@@ -208,7 +208,7 @@ export function SessionManagerPage({ appId }: { appId: string }) {
 
   return (
     <TooltipProvider>
-      <div className="mx-auto px-4 sm:px-6 flex flex-col h-[calc(100vh-8rem)]">
+      <div className="mx-auto px-4 sm:px-6 flex flex-col flex-1 min-h-0">
         <div className="flex-1 overflow-hidden flex flex-col gap-4">
           {/* 主内容区域 - 左右分栏 */}
           <div className="flex-1 overflow-hidden grid gap-4 md:grid-cols-[320px_1fr]">