fix: center header content with symmetric padding

Use pt-3 pb-3 (12px each) for symmetric vertical padding so content
is visually centered in every header bar.

To keep headers draggable, each header container gets the drag-region
class and all interactive elements (buttons) get the no-drag class.
The global fallback drag strip is reduced to h-3.
Title bar overlay reduced from 40px to 32px (standard Windows size).

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

src/renderer/components/ChatPane.tsx

@@ -313,7 +313,7 @@ export function ChatPane({
   return (
     <div className="flex h-full flex-col">
       {/* Header — extra top padding clears the title bar overlay zone */}
-      <header className="border-b border-[var(--color-border)] px-6 pb-3 pt-12">
+      <header className="drag-region border-b border-[var(--color-border)] px-6 pb-3 pt-3">
         <div className="flex min-h-8 items-center justify-between">
           <div className="min-w-0">
             <h2 className="truncate text-[13px] font-semibold leading-tight text-zinc-100">{session.title}</h2>