feat: Expo scaffold + auth screens + core screens (#39-#46, #48)

app/(auth)/_layout.tsx

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+import { Stack } from 'expo-router';
+
+export default function AuthLayout() {
+  return <Stack screenOptions={{ headerShown: false }} />;
+}

app/(auth)/company-code.tsx

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+import { useState } from 'react';
+import { View, Text, TextInput, TouchableOpacity, ActivityIndicator, Alert, KeyboardAvoidingView, Platform } from 'react-native';
+import { router } from 'expo-router';
+import { api } from '../../services/api';
+
+export default function CompanyCodeScreen() {
+  const [slug, setSlug] = useState('');
+  const [loading, setLoading] = useState(false);
+
+  const handleContinue = async () => {
+    if (!slug.trim()) return Alert.alert('Required', 'Please enter your company code.');
+    setLoading(true);
+    try {
+      const res = await api.get(`/api/v1/auth/tenant/${slug.trim().toLowerCase()}/exists`);
+      if (res.data?.exists) {
+        router.push({ pathname: '/(auth)/login', params: { tenantSlug: slug.trim().toLowerCase() } });
+      } else {
+        Alert.alert('Not Found', 'Company code not found. Please check and try again.');
+      }
+    } catch {
+      Alert.alert('Error', 'Could not verify company code. Please try again.');
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <KeyboardAvoidingView
+      className="flex-1 bg-white"
+      behavior={Platform.OS === 'ios' ? 'padding' : 'height'}
+    >
+      <View className="flex-1 justify-center px-8">
+        <View className="mb-10 items-center">
+          <View className="w-16 h-16 rounded-2xl bg-primary items-center justify-center mb-4">
+            <Text className="text-white text-3xl font-bold">F</Text>
+          </View>
+          <Text className="text-3xl font-bold text-gray-900">FiberOps</Text>
+          <Text className="text-gray-500 mt-1">Field Operations</Text>
+        </View>
+
+        <Text className="text-xl font-semibold text-gray-900 mb-2">Enter Company Code</Text>
+        <Text className="text-gray-500 mb-6">Ask your admin for your company's unique code.</Text>
+
+        <TextInput
+          className="border border-gray-300 rounded-xl px-4 py-3 text-base text-gray-900 mb-4"
+          placeholder="e.g. mybusiness"
+          value={slug}
+          onChangeText={setSlug}
+          autoCapitalize="none"
+          autoCorrect={false}
+          autoFocus
+        />
+
+        <TouchableOpacity
+          className="bg-primary rounded-xl py-4 items-center"
+          onPress={handleContinue}
+          disabled={loading}
+        >
+          {loading ? (
+            <ActivityIndicator color="white" />
+          ) : (
+            <Text className="text-white font-semibold text-base">Continue</Text>
+          )}
+        </TouchableOpacity>
+      </View>
+    </KeyboardAvoidingView>
+  );
+}

app/(auth)/login.tsx

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+import { useState } from 'react';
+import { View, Text, TextInput, TouchableOpacity, ActivityIndicator, Alert, KeyboardAvoidingView, Platform } from 'react-native';
+import { useLocalSearchParams, router } from 'expo-router';
+import { useAuthStore } from '../../stores/authStore';
+
+export default function LoginScreen() {
+  const { tenantSlug } = useLocalSearchParams<{ tenantSlug: string }>();
+  const [username, setUsername] = useState('');
+  const [password, setPassword] = useState('');
+  const { login, isLoading } = useAuthStore();
+
+  const handleLogin = async () => {
+    if (!username.trim() || !password) return Alert.alert('Required', 'Please enter username and password.');
+    try {
+      await login(tenantSlug, username.trim(), password);
+      router.replace('/(app)/dashboard');
+    } catch (e: any) {
+      const msg = e?.response?.data?.message ?? 'Login failed. Check your credentials.';
+      Alert.alert('Login Failed', Array.isArray(msg) ? msg.join('\n') : msg);
+    }
+  };
+
+  return (
+    <KeyboardAvoidingView
+      className="flex-1 bg-white"
+      behavior={Platform.OS === 'ios' ? 'padding' : 'height'}
+    >
+      <View className="flex-1 justify-center px-8">
+        <TouchableOpacity className="mb-8" onPress={() => router.back()}>
+          <Text className="text-primary text-base">← Back</Text>
+        </TouchableOpacity>
+
+        <Text className="text-2xl font-bold text-gray-900 mb-1">Welcome back</Text>
+        <Text className="text-gray-500 mb-8">
+          Signing in to <Text className="font-semibold text-gray-700">{tenantSlug}</Text>
+        </Text>
+
+        <Text className="text-sm font-medium text-gray-700 mb-1">Username</Text>
+        <TextInput
+          className="border border-gray-300 rounded-xl px-4 py-3 text-base text-gray-900 mb-4"
+          placeholder="Enter username"
+          value={username}
+          onChangeText={setUsername}
+          autoCapitalize="none"
+          autoCorrect={false}
+          autoFocus
+        />
+
+        <Text className="text-sm font-medium text-gray-700 mb-1">Password</Text>
+        <TextInput
+          className="border border-gray-300 rounded-xl px-4 py-3 text-base text-gray-900 mb-6"
+          placeholder="Enter password"
+          value={password}
+          onChangeText={setPassword}
+          secureTextEntry
+        />
+
+        <TouchableOpacity
+          className="bg-primary rounded-xl py-4 items-center"
+          onPress={handleLogin}
+          disabled={isLoading}
+        >
+          {isLoading ? (
+            <ActivityIndicator color="white" />
+          ) : (
+            <Text className="text-white font-semibold text-base">Sign In</Text>
+          )}
+        </TouchableOpacity>
+      </View>
+    </KeyboardAvoidingView>
+  );
+}