feat(login): campo de dirección del servidor configurable

- config.ts: BASE_URL/ORIGIN dejan de ser constantes; ahora hay
  setServerOrigin/getBaseUrl/getServerOrigin con normalizeServerUrl
  (añade http:// si falta, quita barras finales y el sufijo /api/v1).
- client.ts: construye las URLs con getBaseUrl() en cada petición.
- session: signIn recibe la URL, la fija antes del login y la persiste
  en SecureStore (clave server_url); se rehidrata al arrancar junto al token.
- LoginScreen: campo "Servidor" precargado con el último valor usado.

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

src/screens/LoginScreen.tsx

@@ -15,7 +15,8 @@ import { useSession } from '../auth/session';
 const DEVICE_NAME = `${Platform.OS}-avante`;
 
 export function LoginScreen() {
-  const { signIn } = useSession();
+  const { signIn, serverUrl } = useSession();
+  const [server, setServer] = useState(serverUrl);
   const [email, setEmail] = useState('');
   const [password, setPassword] = useState('');
   const [loading, setLoading] = useState(false);
@@ -25,7 +26,7 @@ export function LoginScreen() {
     setError(null);
     setLoading(true);
     try {
-      await signIn(email.trim(), password, DEVICE_NAME);
+      await signIn(email.trim(), password, DEVICE_NAME, server);
     } catch (e) {
       if (e instanceof ApiError && e.status === 422) {
         setError('Credenciales inválidas.');
@@ -37,6 +38,8 @@ export function LoginScreen() {
     }
   };
 
+  const canSubmit = !loading && !!email && !!password && !!server.trim();
+
   return (
     <KeyboardAvoidingView
       style={styles.container}
@@ -44,6 +47,17 @@ export function LoginScreen() {
     >
       <Text style={styles.title}>Avante · Seguimiento de obra</Text>
 
+      <Text style={styles.fieldLabel}>Servidor</Text>
+      <TextInput
+        style={styles.input}
+        placeholder="https://servidor o IP (p.ej. 192.168.1.50/construprogress/public)"
+        autoCapitalize="none"
+        autoCorrect={false}
+        keyboardType="url"
+        value={server}
+        onChangeText={setServer}
+      />
+
       <TextInput
         style={styles.input}
         placeholder="Email"
@@ -63,9 +77,9 @@ export function LoginScreen() {
       {error && <Text style={styles.error}>{error}</Text>}
 
       <TouchableOpacity
-        style={[styles.button, (loading || !email || !password) && styles.buttonDisabled]}
+        style={[styles.button, !canSubmit && styles.buttonDisabled]}
         onPress={onSubmit}
-        disabled={loading || !email || !password}
+        disabled={!canSubmit}
       >
         {loading ? (
           <ActivityIndicator color="#fff" />
@@ -80,6 +94,7 @@ export function LoginScreen() {
 const styles = StyleSheet.create({
   container: { flex: 1, justifyContent: 'center', padding: 24, gap: 12 },
   title: { fontSize: 22, fontWeight: '700', textAlign: 'center', marginBottom: 16 },
+  fieldLabel: { fontSize: 12, color: '#666', marginBottom: -6, fontWeight: '600' },
   input: {
     borderWidth: 1,
     borderColor: '#ccc',