fix(app): keep header height constant with or without back button

src/App.tsx

@@ -39,8 +39,8 @@ function App() {
   return (
     <main className="mx-auto flex w-full max-w-2xl min-h-dvh flex-col pb-[calc(env(safe-area-inset-bottom)+7rem)]">
       <header className="sticky top-0 z-10 border-b border-gray-200 bg-white/95 px-4 pb-3 pt-[calc(env(safe-area-inset-top)+0.75rem)] backdrop-blur">
+        <div className="flex h-11 items-center gap-1">
             {creating || selectedNote ? (
-          <div className="flex items-center gap-1">
               <button
                 type="button"
                 onClick={goBack}
@@ -49,11 +49,9 @@ function App() {
               >
                 ←
               </button>
+            ) : null}
             <h1 className="text-lg font-semibold">Notes</h1>
           </div>
-        ) : (
-          <h1 className="text-lg font-semibold">Notes</h1>
-        )}
       </header>
 
       <section className="flex-1 px-4 py-4">