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/App.tsx

@@ -752,7 +752,7 @@ function App() {
           return <AgentsDefaultsPanel />;
         default:
           return (
-            <div className="px-6 flex flex-col h-[calc(100vh-8rem)] overflow-hidden">
+            <div className="px-6 flex flex-col flex-1 min-h-0 overflow-hidden">
               <div className="flex-1 overflow-y-auto overflow-x-hidden pb-12 px-1">
                 <AnimatePresence mode="wait">
                   <motion.div