frontend: add Create-profile setup screen for brand-new accounts

LoginGate now shows CreateProfile (display name only) instead of App when
an authenticated session has no provisioned geniusrun profile yet, backed
by the new POST /api/setup endpoint and session/me's has_profile flag.

frontend/src/CreateProfile.tsx

@@ -0,0 +1,52 @@
+import { useState } from "react";
+import { api } from "./api/client";
+import "./CreateProfile.css";
+
+// Shown once, right after a brand-new OIDC login, before the account has a
+// geniusrun profile at all. The only thing asked for is a display name --
+// Garmin credentials and every other tunable are filled in afterward via
+// the existing Profile screen, same as a fresh single-user install today.
+export function CreateProfile({ onCreated }: { onCreated: (displayName: string) => void }) {
+  const [displayName, setDisplayName] = useState("");
+  const [submitting, setSubmitting] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+    const trimmed = displayName.trim();
+    if (!trimmed) {
+      setError("Please enter a display name.");
+      return;
+    }
+    setSubmitting(true);
+    setError(null);
+    try {
+      const result = await api.setup(trimmed);
+      onCreated(result.display_name);
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Something went wrong, please try again.");
+      setSubmitting(false);
+    }
+  };
+
+  return (
+    <div className="create-profile">
+      <h1>🧞‍♀️ Welcome to geniusrun</h1>
+      <p>Let's set up your profile. You can add your Garmin account afterward.</p>
+      <form onSubmit={handleSubmit}>
+        <input
+          type="text"
+          placeholder="Display name"
+          value={displayName}
+          onChange={(e) => setDisplayName(e.target.value)}
+          disabled={submitting}
+          autoFocus
+        />
+        {error && <p className="create-profile-error">{error}</p>}
+        <button type="submit" disabled={submitting}>
+          {submitting ? "Creating…" : "Continue"}
+        </button>
+      </form>
+    </div>
+  );
+}