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/prompts/PromptPanel.tsx

@@ -96,7 +96,7 @@ const PromptPanel = React.forwardRef<PromptPanelHandle, PromptPanelProps>(
     const enabledPrompt = promptEntries.find(([_, p]) => p.enabled);
 
     return (
-      <div className="flex flex-col h-[calc(100vh-8rem)] px-6">
+      <div className="flex flex-col flex-1 min-h-0 px-6">
         <div className="flex-shrink-0 py-4 glass rounded-xl border border-white/10 mb-4 px-6">
           <div className="text-sm text-muted-foreground">
             {t("prompts.count", { count: promptEntries.length })} ·{" "}