refactor: import React types directly instead of namespace

Replace 'import React from "react"' with direct imports of
CSSProperties and ChangeEvent. This eliminates unused imports
and follows modern React patterns where the namespace import
is not required for JSX (React 17+).

frontend/app/components/SatsDisplay.tsx

@@ -1,4 +1,4 @@
-import React from "react";
+import { CSSProperties } from "react";
 
 /**
  * Format satoshi amount with styled components.
@@ -26,7 +26,7 @@ export function SatsDisplay({ sats }: { sats: number }) {
   }
 
   // Build the display with subtle leading zeros and prominent digits
-  const subtleStyle: React.CSSProperties = {
+  const subtleStyle: CSSProperties = {
     opacity: 0.45,
     fontWeight: 400,
   };