update copy for sign in views

This disambiguates whether the view is for sign up or sign in.

js/desktop/src/features/auth/email-step.tsx

@@ -31,8 +31,8 @@ export function EmailStep({ onCodeSent }: EmailStepProps) {
   return (
     <form onSubmit={handleSubmit} className="flex flex-col gap-4">
       <div className="flex flex-col gap-2">
-        <H3>Sign in</H3>
-        <Muted>Enter your email to receive a sign-in code.</Muted>
+        <H3>Welcome</H3>
+        <Muted>Enter your email to sign in or create an account. We'll send a code to verify it.</Muted>
       </div>
 
       <div className="flex flex-col gap-1.5">

js/mobile/src/features/auth/SignInScreen.tsx

@@ -60,9 +60,9 @@ function EmailStep({ onCodeSent }: { onCodeSent: (email: string) => void }) {
   return (
     <View className="gap-5">
       <View className="gap-1">
-        <Text className="text-foreground text-3xl font-semibold">Sign in</Text>
+        <Text className="text-foreground text-3xl font-semibold">Welcome</Text>
         <Text className="text-muted-foreground text-base">
-          Enter your email to receive a sign-in code.
+          Enter your email to sign in or create an account. We'll send a code to verify it.
         </Text>
       </View>