Handle deeplinks fallback mode

pages/_app.tsx

@@ -19,7 +19,8 @@ const AUTHORIZED_UNLOGGED_URLS = [
   '/signin',
   '/signup',
   '/reset-password',
-  '/auth/reset-password'
+  '/auth/reset-password',
+  '/app'
 ]
 
 function App ({ Component, pageProps }: AppProps): React.ReactElement | null {

pages/app/index.tsx

@@ -0,0 +1,96 @@
+import React, { useEffect } from 'react'
+import { GetServerSideProps } from 'next'
+import { parseAddress } from 'utils/validators'
+
+const convertToBip21 = (queryParams: Record<string, string | string[] | undefined>): string | null => {
+  // Extract and validate address parameter (required)
+  let parsedAddress: string
+  try {
+    const address = queryParams.address
+    if (typeof address !== 'string') {
+      return null
+    }
+    parsedAddress = parseAddress(address)
+  } catch {
+    return null
+  }
+
+  // Build query string from all parameters except 'address' and 'b'
+  const queryParts: string[] = []
+  for (const [key, value] of Object.entries(queryParams)) {
+    // Skip 'address' and 'b' parameters
+    if (key === 'address' || key === 'b') {
+      continue
+    }
+
+    // Handle array values (take first element) or string values
+    const paramValue = Array.isArray(value) ? value[0] : value
+    if (paramValue !== undefined && paramValue !== '') {
+      queryParts.push(`${key}=${paramValue}`)
+    }
+  }
+
+  // Construct BIP21 string
+  if (queryParts.length > 0) {
+    return `${parsedAddress}?${queryParts.join('&')}`
+  }
+  return parsedAddress
+}
+
+export const getServerSideProps: GetServerSideProps = async (context) => {
+  const queryParams = context.query
+
+  // Convert to BIP21 string (validates address internally)
+  const bip21String = convertToBip21(queryParams)
+  if (bip21String === null) {
+    context.res.statusCode = 400
+    return {
+      props: {
+        error: 'Invalid PayButton URL.'
+      }
+    }
+  }
+
+  return {
+    props: {
+      bip21String
+    }
+  }
+}
+
+interface AppProps {
+  bip21String?: string
+  error?: string
+}
+
+export default function App ({ bip21String, error }: AppProps): JSX.Element {
+  useEffect(() => {
+    if (error !== undefined || bip21String === undefined) {
+      return
+    }
+
+    window.location.href = `https://cashtab.com/#/send?bip21=${bip21String}`
+  }, [bip21String, error])
+
+  if (error !== undefined) {
+    return (
+      <div>
+        <h1>Error</h1>
+        <p>{error}</p>
+      </div>
+    )
+  }
+
+  // bip21String should always be defined at that point, but this makes eslint
+  // happy and hardens the code a bit.
+  const cashtabRedirectUrl = typeof bip21String === 'string'
+    ? `https://cashtab.com/#/send?bip21=${bip21String}`
+    : 'https://cashtab.com'
+
+  return (
+    <div>
+      <h1>Redirecting to Cashtab...</h1>
+      <p>If you are not redirected, please click <a href={cashtabRedirectUrl}>here</a>.</p>
+    </div>
+  )
+}