fix(polish): remove identity card kicker, flatten to success border

- Remove 'Active signing identity' kicker text
- Card uses flat surface background with var(--success) border
  (matches active profile card treatment)
- Remove dead .home-identity-kicker CSS
- Update test to find card by 'Active profile' heading

frontend/src/screens/HomeScreen.tsx

@@ -81,7 +81,6 @@ export function HomeScreen({ onNavigate, onCreateProfile }: HomeScreenProps) {
             <div className="home-identity-content">
               <Avatar npub={active.npub} label={active.label} size="lg" picture={active.picture} />
               <div className="home-identity-copy">
-                <p className="home-identity-kicker">Active signing identity</p>
                 <h2 id="active-identity-title">Active profile</h2>
                 <strong className="home-identity-name" aria-label="Active profile name">
                   {active.label}

frontend/src/styles.css

@@ -1480,7 +1480,7 @@ select {
   justify-content: space-between;
   gap: 20px;
   border-color: var(--success);
-  background: var(--success-soft);
+  background: var(--surface);
 }
 
 .home-identity-content {
@@ -1494,15 +1494,6 @@ select {
   min-width: 0;
 }
 
-.home-identity-kicker {
-  margin: 0 0 4px;
-  color: var(--success);
-  font-size: 12px;
-  font-weight: 650;
-  letter-spacing: 0.08em;
-  text-transform: uppercase;
-}
-
 .home-identity-copy h2 {
   margin: 0 0 4px;
 }

frontend/src/test/HomeScreen.test.tsx

@@ -24,7 +24,7 @@ describe('HomeScreen', () => {
     renderWithApp(<HomeScreen onNavigate={onNavigate} onCreateProfile={vi.fn()} />);
 
     // The active profile name appears both in the identity card and in the profile row.
-    const identityCard = (await screen.findByText('Active signing identity')).closest(
+    const identityCard = (await screen.findByText('Active profile')).closest(
       '.home-identity-card',
     ) as HTMLElement;
     expect(within(identityCard).getByText('Alice')).toBeInTheDocument();