fix(rcart): force Transak dark mode with colorMode param

Text inputs were rendering in light mode making text invisible against
the dark background. Added colorMode: 'DARK' to both Transak widget
endpoints.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/rcart/mod.ts

@@ -1781,6 +1781,7 @@ routes.post("/api/payments/:id/transak-session", async (c) => {
 		partnerOrderId: `pay-${paymentId}`,
 		email,
 		themeColor: '6366f1',
+		colorMode: 'DARK',
 		hideMenu: 'true',
 	};
 
@@ -1858,6 +1859,7 @@ routes.post("/api/payments/:id/card-session", async (c) => {
 		partnerOrderId: `pay-${paymentId}`,
 		email,
 		themeColor: '6366f1',
+		colorMode: 'DARK',
 		hideMenu: 'true',
 	};
 	// Derive fiat amount: use explicit fiatAmount, or infer from crypto amount for stablecoins