Detect checkout email collisions and show login state in the navbar

Registering with an email that already has an account previously just
failed with a generic error and no clear next step. registerCustomer()
now flags emailExists specifically, and checkout switches straight to the
login toggle (email pre-filled, scrolled into view) instead. The account
icon also gets a small underline while logged in, matching the nav links'
active-state styling — it was otherwise the only nav element that gave no
visual signal of session state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/customerAuth.ts

@@ -39,7 +39,9 @@ export type CustomerSummary = {
   emailVerified: boolean;
 };
 
-export type AuthResult = { ok: true; token: string; customer: CustomerSummary } | { ok: false; reason: string };
+export type AuthResult =
+  | { ok: true; token: string; customer: CustomerSummary }
+  | { ok: false; reason: string; emailExists?: boolean };
 
 export async function registerCustomer(input: {
   firstName: string;
@@ -58,7 +60,23 @@ export async function registerCustomer(input: {
   if (!res.ok) {
     const data = await res.json().catch(() => null);
     const message: string | undefined = data?.errors?.[0]?.message;
-    return { ok: false, reason: message ?? "Diese E-Mail-Adresse ist bereits registriert." };
+    // Payload's own message for a duplicate unique field is a generic
+    // "The following field is invalid: email" — not distinguishable from
+    // any other email-field validation failure by content alone, but at
+    // this point the client's own type="email" + required already ruled
+    // out a malformed/missing address, so "email" being the flagged field
+    // here in practice only ever means one thing: this address is already
+    // registered. emailExists lets the checkout UI react to that
+    // specifically (switch to the login toggle) instead of just showing
+    // an error the customer has no clear next step for.
+    const emailExists = Boolean(message?.toLowerCase().includes("email"));
+    return {
+      ok: false,
+      reason: emailExists
+        ? "Diese E-Mail-Adresse ist bereits registriert. Bitte logge dich stattdessen ein."
+        : (message ?? "Registrierung ist gerade nicht möglich."),
+      emailExists,
+    };
   }
 
   return loginCustomer(input);