fix: align chat and activity panel header borders

Reduce chat header padding and tighten the title/subtitle line height so the border-bottom lines up with the activity side panel header.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/ActivityPanel.tsx

@@ -46,7 +46,7 @@ export function ActivityPanel({ activity, pattern, session }: ActivityPanelProps
     <div className="flex h-full flex-col">
       {/* Header — top padding clears the title bar overlay zone */}
       <div className="border-b border-[var(--color-border)] px-4 pb-3 pt-12">
-        <div className="flex items-center gap-2">
+        <div className="flex min-h-8 items-center gap-2">
           <Activity className="size-4 text-zinc-500" />
           <span className="text-[12px] font-semibold uppercase tracking-[0.12em] text-zinc-400">
             Activity