design(auth): update session loading state to match new premium dark mode aesthetic

vibn-frontend/app/auth/page.tsx

@@ -30,10 +30,47 @@ function AuthPageInner() {
 
   if (status === "loading") {
     return (
-      <div className="justine-auth-inner">
-        <div className="justine-auth-loading-wrap">
-          <div className="justine-auth-spinner" aria-hidden />
-          <p className="justine-auth-loading-text">Loading authentication…</p>
+      <div
+        className="new-site-wrapper"
+        style={{
+          display: "flex",
+          alignItems: "center",
+          justifyContent: "center",
+          minHeight: "100vh",
+        }}
+      >
+        <div
+          style={{
+            display: "flex",
+            flexDirection: "column",
+            alignItems: "center",
+            gap: "16px",
+          }}
+        >
+          <div
+            style={{
+              width: "24px",
+              height: "24px",
+              borderRadius: "50%",
+              border: "2px solid oklch(0.20 0.009 60)",
+              borderTopColor: "var(--accent)",
+              animation: "spin .9s linear infinite",
+            }}
+          />
+          <style>{`
+            @keyframes spin { to { transform: rotate(360deg); } }
+          `}</style>
+          <div
+            style={{
+              color: "var(--fg-mute)",
+              fontFamily: "var(--font-mono)",
+              fontSize: "11px",
+              letterSpacing: "0.1em",
+              textTransform: "uppercase",
+            }}
+          >
+            Checking session
+          </div>
         </div>
       </div>
     );