fix: download option for unsupported OS

src/components/download.tsx

@@ -579,7 +579,7 @@ export default function DownloadPage() {
 							</Button>
 							<Button
 								onClick={() => continueFlow()}
-								disabled={selectedPlatform === null}
+								disabled={selectedPlatform === ""}
 							>
 								{(flowIndex === 1 && platform === "MacOS") || flowIndex === 2
 									? "Download 🥳"
@@ -587,6 +587,15 @@ export default function DownloadPage() {
 							</Button>
 						</div>
 					)}
+					{selectedPlatform === "" && (
+						<div className="mt-5 flex items-center">
+							<InfoCircledIcon className="mr-2 size-4" />
+							<p className="text-muted-foreground">
+								Unfortunately, Zen Browser is not available for your platform at
+								this time.
+							</p>
+						</div>
+					)}
 					{(platform === "Linux" || platform === "Windows") &&
 						flowIndex === 1 && (
 							<div className="mt-5 flex items-center">