style(app): mobile-first layout polish

src/App.tsx

@@ -28,15 +28,15 @@ function App() {
   }
 
   return (
-    <main className="mx-auto w-full max-w-2xl flex flex-col min-h-screen">
-      <header className="sticky top-0 bg-white border-b border-gray-200 px-4 py-3">
+    <main className="mx-auto w-full max-w-2xl flex flex-col min-h-dvh pb-28">
+      <header className="sticky top-0 z-10 bg-white/95 backdrop-blur border-b border-gray-200 px-4 py-3">
         {selectedNote ? (
-          <div className="flex items-center gap-2">
+          <div className="flex items-center gap-1">
             <button
               type="button"
               onClick={() => setSelectedId(null)}
               aria-label="Retour"
-              className="w-10 h-10 rounded-full text-xl hover:bg-gray-100"
+              className="flex items-center justify-center w-11 h-11 -ml-2 rounded-full text-xl text-gray-700 hover:bg-gray-100 active:bg-gray-200"
             >
               ←
             </button>