refactor(frontend): migrate Profile.tsx to the banner system

Load/save/delete failures now call showError instead of setting local
error state. The profile-not-loaded-yet fallback gains a
profileLoadFailed boolean so it can still distinguish "loading" from
"failed to load" without the error text itself, which now lives only
in the (transient) banner.

frontend/src/pages/Profile.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState } from "react";
 import { api, BASE_URL } from "../api/client";
+import { showError } from "../banner";
 import { ColorField } from "../components/ColorField";
 import { GarminConnection } from "../components/GarminConnection";
 import { NullableNumberField } from "../components/NullableNumberField";
@@ -87,7 +88,7 @@ const AUTO_SAVE_DELAY_MS = 600;
 
 export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
   const [profile, setProfile] = useState<ProfileType | null>(null);
-  const [error, setError] = useState<string | null>(null);
+  const [profileLoadFailed, setProfileLoadFailed] = useState(false);
   const [saved, setSaved] = useState(false);
   // Mirrors `profile` synchronously (state updates don't apply until the
   // next render), so set() always debounces from the latest edit rather
@@ -100,7 +101,10 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
   const [deleting, setDeleting] = useState(false);
 
   useEffect(() => {
-    api.getProfile().then(setProfile).catch((e) => setError(String(e)));
+    api.getProfile().then(setProfile).catch((e) => {
+      showError(String(e));
+      setProfileLoadFailed(true);
+    });
   }, []);
 
   // Flush a still-pending debounced save on unmount (e.g. the user edits a
@@ -121,11 +125,10 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
       const updated = await api.updateProfile(next);
       profileRef.current = updated;
       setProfile(updated);
-      setError(null);
       setSaved(true);
       onSaved?.(updated);
     } catch (e) {
-      setError(String(e));
+      showError(String(e));
     }
   }
 
@@ -153,7 +156,6 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
 
   async function deleteAccount() {
     setDeleting(true);
-    setError(null);
     try {
       await api.deleteProfile();
       // Deletion doesn't clear the session cookie -- follow with a real
@@ -166,7 +168,7 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
       document.body.appendChild(form);
       form.submit();
     } catch (e) {
-      setError(String(e));
+      showError(String(e));
       setDeleting(false);
     }
   }
@@ -174,14 +176,13 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
   if (!profile) {
     return (
       <div className="page">
-        {error ? <p className="error">{error}</p> : <p>Loading...</p>}
+        {profileLoadFailed ? <p className="empty-state">Couldn't load profile.</p> : <p>Loading...</p>}
       </div>
     );
   }
 
   return (
     <div className="page profile-page">
-      {error && <p className="error">{error}</p>}
       {saved && <p className="garmin-connection-message">Saved.</p>}
 
       <fieldset className="kind-editor">