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>

README.md

@@ -287,7 +287,21 @@ without leaving the page.
   server-rendered root layout) specifically so `app/layout.tsx` — otherwise
   static/ISR-cacheable — doesn't get forced into per-request dynamic
   rendering just to know one icon's href; briefly shows the logged-out
-  state on first paint until that fetch resolves.
+  state on first paint until that fetch resolves. The icon itself also
+  gets a small brand-colored underline while logged in — same visual
+  language as the desktop nav links' active-state indicator — since the
+  icon alone doesn't otherwise signal session state at a glance.
+- **Checkout registration collisions**: if the email typed into Card 1
+  during inline registration already belongs to an existing account,
+  Payload's create call fails — `registerCustomer()` in `customerAuth.ts`
+  detects this specifically (`emailExists: true` on the returned
+  `AuthResult`, inferred from the field flagged in Payload's validation
+  error, since Payload's own message text doesn't distinguish "duplicate"
+  from other email-field failures) rather than just surfacing a generic
+  error. `CheckoutContent.tsx`'s `handleSubmit` reacts by switching
+  straight to the login toggle with that email pre-filled and
+  scroll-into-view, instead of leaving the customer stuck with an error
+  and no obvious next step.
 - **`/konto/profil`** edits name + the one saved default address (deliberately
   a single address, not a full address book — see the assistant's memory
   note on optionally expanding this later), changes the password, shows