Fix nested scrollbar issue on keys page, add local data proxy

- Change root layout from min-h-screen to h-screen with overflow-hidden
  to properly constrain viewport height
- Add min-h-0 to main element to allow flex shrinking
- Add rewrite rule to proxy /data/* to production data URL for local dev
- Make output: "export" conditional (production only) since rewrites
  require server mode

src/app/layout.tsx

@@ -35,11 +35,11 @@ export default function RootLayout({
         className={`${geistSans.variable} ${geistMono.variable} antialiased`}
       >
         <ThemeProvider attribute="class" defaultTheme="system" enableSystem>
-          <div className="flex flex-col min-h-screen">
+          <div className="flex flex-col h-screen overflow-hidden">
             <NavTop />
             <Toaster />
             <Suspense>
-              <main className="flex-1 flex flex-col">{children}</main>
+              <main className="flex-1 flex flex-col min-h-0">{children}</main>
             </Suspense>
             <Footer />
           </div>