fix: add auth endpoints to server, fix auth bypass and registration

- server/: commit server code for the first time (was untracked)
  - POST /auth/signup and /auth/login endpoints now deployed
  - GET /v1/billing/summary now verifies user exists in auth_users
    (prevents stale JWTs from bypassing auth → fixes empty dashboard)
- app/_layout.tsx: dual-marker install check (SQLite + SecureStore)
  to detect fresh installs reliably on Android
- app/auth/login.tsx, signup.tsx: replace Ionicons leaf logo with
  actual app icon image (assets/icon.png)
- services/authService.ts: log HTTP status + server message on auth errors

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

app/_layout.tsx

@@ -0,0 +1,118 @@
+import { useEffect, useState } from 'react';
+import { Stack } from 'expo-router';
+import { StatusBar } from 'expo-status-bar';
+import { StripeProvider } from '@stripe/stripe-react-native';
+import AsyncStorage from '@react-native-async-storage/async-storage';
+import { AppProvider, useApp } from '../context/AppContext';
+import { CoachMarksProvider } from '../context/CoachMarksContext';
+import { CoachMarksOverlay } from '../components/CoachMarksOverlay';
+import { useColors } from '../constants/Colors';
+import { AuthService } from '../services/authService';
+import { initDatabase, AppMetaDb } from '../services/database';
+import * as SecureStore from 'expo-secure-store';
+
+type InitialRoute = 'onboarding' | 'auth/login' | '(tabs)';
+const STRIPE_PUBLISHABLE_KEY = (process.env.EXPO_PUBLIC_STRIPE_PUBLISHABLE_KEY || 'pk_test_mock_key').trim();
+const SECURE_INSTALL_MARKER = 'greenlens_install_v1';
+
+const ensureInstallConsistency = async (): Promise<boolean> => {
+  try {
+    const [sqliteMarker, secureMarker] = await Promise.all([
+      Promise.resolve(AppMetaDb.get('install_marker_v2')),
+      SecureStore.getItemAsync(SECURE_INSTALL_MARKER).catch(() => null),
+    ]);
+
+    if (sqliteMarker && secureMarker) return false; // Kein Fresh Install
+
+    // Fresh Install: Alles zurücksetzen
+    await AuthService.logout();
+    await AsyncStorage.removeItem('greenlens_show_tour');
+    AppMetaDb.set('install_marker_v2', '1');
+    await SecureStore.setItemAsync(SECURE_INSTALL_MARKER, '1');
+    return true;
+  } catch (error) {
+    console.error('Failed to initialize install marker', error);
+    return false;
+  }
+};
+
+function RootLayoutInner() {
+  const { isDarkMode, colorPalette, signOut } = useApp();
+  const colors = useColors(isDarkMode, colorPalette);
+  const [initialRoute, setInitialRoute] = useState<InitialRoute | null>(null);
+
+  useEffect(() => {
+    (async () => {
+      const didResetSessionForFreshInstall = await ensureInstallConsistency();
+      if (didResetSessionForFreshInstall) {
+        await signOut();
+      }
+      const session = await AuthService.getSession();
+      if (!session) {
+        // Kein Benutzer → immer zum Onboarding (Landing + Register/Login)
+        setInitialRoute('auth/login');
+        return;
+      }
+      const validity = await AuthService.validateWithServer();
+      if (validity === 'invalid') {
+        await AuthService.logout();
+        await signOut();
+        setInitialRoute('auth/login');
+        return;
+      }
+      // 'valid' or 'unreachable' (offline) → allow tab navigation
+      setInitialRoute('(tabs)');
+    })();
+  }, [signOut]);
+
+  if (initialRoute === null) return null;
+
+  return (
+    <>
+      <StatusBar style={isDarkMode ? 'light' : 'dark'} />
+      <Stack
+        screenOptions={{
+          headerShown: false,
+          contentStyle: { backgroundColor: colors.background },
+        }}
+        initialRouteName={initialRoute}
+      >
+        <Stack.Screen name="onboarding" options={{ animation: 'none' }} />
+        <Stack.Screen name="auth/login" options={{ animation: 'slide_from_right' }} />
+        <Stack.Screen name="auth/signup" options={{ animation: 'slide_from_right' }} />
+        <Stack.Screen name="(tabs)" options={{ animation: 'none' }} />
+        <Stack.Screen
+          name="scanner"
+          options={{ presentation: 'fullScreenModal', animation: 'slide_from_bottom' }}
+        />
+        <Stack.Screen
+          name="plant/[id]"
+          options={{ presentation: 'card', animation: 'slide_from_right' }}
+        />
+        <Stack.Screen
+          name="lexicon"
+          options={{ presentation: 'fullScreenModal', animation: 'slide_from_bottom' }}
+        />
+      </Stack>
+      {/* Coach Marks rendern über allem */}
+      <CoachMarksOverlay />
+    </>
+  );
+}
+
+export default function RootLayout() {
+  initDatabase();
+
+  return (
+    <StripeProvider
+      publishableKey={STRIPE_PUBLISHABLE_KEY}
+      merchantIdentifier="merchant.com.greenlens"
+    >
+      <AppProvider>
+        <CoachMarksProvider>
+          <RootLayoutInner />
+        </CoachMarksProvider>
+      </AppProvider>
+    </StripeProvider>
+  );
+}