fix: update appbar decoration to match khalti brand

[iamnabink]

Currently payment page has white non-visible appbar which needs to be changed to something visible so added default khalti brand color.

[mrdeephang]

@iamnabink
The PR mentions matching Khalti branding, but no actual brand color is used. The hardcoded Color(0xFFE8F0F7) also caused visibility issues. It’s better to rely on the app’s theme or use an official brand color of Khalti(Color(0xFFDC0019)) with foregroundColor: Colors.white.
So my proposed change is:

1. //uses khalti brand color
   title: const Text(s_payWithKhalti),
   backgroundColor: Color(0xFFDC0019),
   foregroundColor: Colors.white,
   actions: [
   IconButton(
   onPressed: _reload,
   icon: const Icon(Icons.refresh),
   )
   ],
   elevation: 4,
   ),

2. //uses your app brand Color
   title: const Text(s_payWithKhalti),
   actions: [
   IconButton(
   onPressed: _reload,
   icon: const Icon(Icons.refresh),
   )
   ],
   elevation: 4,
   ),
   Use the app’s brand color via AppBarTheme defined in MaterialApp instead of hardcoding colors.
   appBarTheme: const AppBarTheme(
   centerTitle: true,
   foregroundColor: Colors.white,
   backgroundColor: primaryColor,
   elevation: 0,
   ),