[fix](client): sign in with the email the hub identifies, not a username

Sign-in labels the single identity field 'Username', but `login()` sends it as
`email` and dcntrlized-auth resolves accounts by the email column. Typing the
username of a seeded account ('admin') therefore fails with 'invalid
credentials' while the password is correct. The field is now labelled Email,
with an email keyboard and a matching placeholder, in sign-in mode only —
registration still has a separate username and email.

src/screens/LoginScreen.tsx

@@ -72,14 +72,24 @@ export default function LoginScreen() {
               </AppText>
             </View>
 
-            <Field label="Username" accessibilityLabel="Username">
+            {/* Sign-in identifies the account by EMAIL: the auth hub's identity
+                is the email column, and `login()` sends this value as `email`.
+                Labelling it "Username" made the seeded accounts unusable —
+                typing the username from the seed file fails with "invalid
+                credentials" even though the password is right. Registration is
+                the only flow that has both a username and an email. */}
+            <Field
+              label={isRegister ? 'Username' : 'Email'}
+              accessibilityLabel={isRegister ? 'Username' : 'Email'}
+            >
               <TextInputBox
                 value={username}
                 onChangeText={setUsername}
                 autoCapitalize="none"
                 autoCorrect={false}
-                accessibilityLabel="Username"
+                keyboardType={isRegister ? 'default' : 'email-address'}
-                placeholder="your username"
+                accessibilityLabel={isRegister ? 'Username' : 'Email'}
+                placeholder={isRegister ? 'your username' : '[email protected]'}
               />
             </Field>