fix(sub): restore standard base64 for Shadowrocket sub link (#5001)

URL-safe base64 (-/_ with stripped padding) broke Shadowrocket import: it decodes the add/sub path segment as standard base64 and rejects -/_, so the subscription was silently not added. Revert to plain btoa() output as originally shipped in #3489.

frontend/src/pages/sub/SubPage.tsx

@@ -120,7 +120,7 @@ export default function SubPage() {
     if (!subUrl) return '';
     const separator = subUrl.includes('?') ? '&' : '?';
     const rawUrl = subUrl + separator + 'flag=shadowrocket';
-    const base64Url = btoa(rawUrl).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+    const base64Url = btoa(rawUrl);
     const remark = encodeURIComponent(subTitle || sId || 'Subscription');
     return `shadowrocket://add/sub/${base64Url}?remark=${remark}`;
   }, []);