fix: restore authentik trusted origin and remove auth card U-border shadow

- Auto-derive trusted origin from AUTHENTIK_ISSUER URL so OAuth callbacks
  are accepted without requiring a separate AUTHENTIK_ORIGIN env var
- Remove leftover ssoProvider schema mapping (no longer used with genericOAuth)
- Remove dead @better-auth/sso dependency from package.json
- Drop md:shadow-2xl/md:shadow-lg from auth cards — the downward box-shadow
  was rendering as a U-shaped border (bottom+sides, no top); border +
  backdrop-blur-xl provides sufficient visual separation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/auth/register/page.tsx

@@ -48,7 +48,7 @@ function RegisterForm() {
 
   return (
     <div className="bg-background flex min-h-screen items-center justify-center">
-      <Card className="mx-auto h-screen w-full overflow-hidden border-0 shadow-none md:h-auto md:max-w-6xl md:border md:shadow-lg">
+      <Card className="mx-auto h-screen w-full overflow-hidden border-0 shadow-none md:h-auto md:max-w-6xl md:border">
         <CardContent className="grid h-full p-0 md:grid-cols-2">
           {/* Hero Section - Hidden on mobile */}
           <div className="bg-muted relative hidden md:flex md:flex-col md:justify-center md:p-12">

src/app/auth/signin/signin-form.tsx

@@ -77,7 +77,7 @@ export function SignInForm({ allowRegistration }: SignInFormProps) {
         <div className="animate-blob h-[800px] w-[800px] rounded-full bg-neutral-400/30 blur-3xl dark:bg-neutral-500/20"></div>
       </div>
 
-      <Card className="md:bg-background/80 md:border-border/50 mx-auto h-screen w-full overflow-hidden border-0 shadow-none md:h-auto md:max-w-6xl md:rounded-3xl md:border md:shadow-2xl md:backdrop-blur-xl">
+      <Card className="md:bg-background/80 md:border-border/50 mx-auto h-screen w-full overflow-hidden border-0 shadow-none md:h-auto md:max-w-6xl md:rounded-3xl md:border md:backdrop-blur-xl">
         <CardContent className="grid h-full p-0 md:grid-cols-2">
           {/* Hero Section - Hidden on mobile */}
           <div className="bg-primary/5 border-border/50 relative hidden border-r md:flex md:flex-col md:justify-center md:p-12">