fix: use redirect=null for mobile BankID to prevent new tab (#194)

Previously the mobile deep link used the current URL as redirect,
causing BankID to open a new browser tab after auth instead of
returning to the original. Now uses redirect=null so the user
switches back manually and the polling picks up completion.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

components/auth/BankIdAuth.tsx

@@ -88,10 +88,10 @@ export function BankIdAuth({ mode, onComplete }: BankIdAuthProps) {
       const newSession: BankIdSession = data
       setSession(newSession)
 
-      // On mobile, open BankID app
+      // On mobile, open BankID app — redirect=null so it doesn't open a new tab;
+      // the polling in this tab detects completion when the user switches back.
       if (isMobile()) {
-        const returnUrl = encodeURIComponent(window.location.href)
-        window.location.href = `bankid:///?autostarttoken=${newSession.autoStartToken}&redirect=${returnUrl}`
+        window.location.href = `bankid:///?autostarttoken=${newSession.autoStartToken}&redirect=null`
       }
 
       // Start polling