Fix login error alert disappearing due to browser autofill

After a failed login, the browser's password manager fires onChange
events on the username/password inputs (clearing or resetting them).
The onChange handlers were calling setLoginError(null), which wiped
the error alert immediately after it appeared.

Fix: remove setLoginError(null) from input onChange handlers. The
error now clears at the start of the next submit attempt via the
existing setLoginError(null) in handleCredentialSubmit.

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

frontend/src/components/auth/LockScreen.tsx

@@ -496,7 +496,7 @@ export default function LockScreen() {
               id="username"
               type="text"
               value={username}
-              onChange={(e) => { setUsername(e.target.value); setLoginError(null); }}
+              onChange={(e) => setUsername(e.target.value)}
               placeholder="Enter username"
               required
               autoFocus
@@ -509,7 +509,7 @@ export default function LockScreen() {
               id="password"
               type="password"
               value={password}
-              onChange={(e) => { setPassword(e.target.value); setLoginError(null); }}
+              onChange={(e) => setPassword(e.target.value)}
               placeholder={isSetup ? 'Create a password' : 'Enter password'}
               required
               autoComplete={isSetup ? 'new-password' : 'current-password'}