feat: replace payment modals with page-based navigation

- Add /balance/top-up route for payment method selection
- Add /balance/top-up/:methodId route for amount entry and payment
- Remove TopUpModal component (619 lines)
- Simplify InsufficientBalancePrompt to use navigate instead of modals
- Support ?amount and ?returnTo query params for cross-page data flow
- Fix nav isActive to highlight Balance on sub-routes
- Remove unused MainButton platform abstraction
- Increase toast accent opacity for better visibility

src/components/layout/AppShell/AppHeader.tsx

@@ -145,7 +145,10 @@ export function AppHeader({
     };
   }, [mobileMenuOpen]);
 
-  const isActive = (path: string) => location.pathname === path;
+  const isActive = (path: string) => {
+    if (path === '/') return location.pathname === '/';
+    return location.pathname.startsWith(path);
+  };
   const isAdminActive = () => location.pathname.startsWith('/admin');
 
   const navItems = [