feat(auth): add user identity verification system with new screens

Add comprehensive identity verification flow including:
- New verification guide screen at /verification-guide for selecting identity type
- New verification form screen at /verification-form for submitting verification documents
- Verification settings screen at /profile/verification for managing verification status
- Move terms and privacy policy to root-level routes (/terms, /privacy)
- Update login to require active terms agreement checkbox
- Database: add in-memory fallback when OPFS is unavailable on web

BREAKING CHANGE: Terms and privacy policy routes moved from /profile/terms and /profile/privacy to /terms and /privacy

app/_layout.tsx

@@ -175,6 +175,30 @@ function ThemedStack() {
           <Stack.Screen name="index" options={{ headerShown: false }} />
           <Stack.Screen name="(auth)" options={{ headerShown: false }} />
           <Stack.Screen name="(app)" options={{ headerShown: false }} />
+          <Stack.Screen
+            name="terms"
+            options={{
+              headerShown: true,
+              title: '用户协议',
+              headerStyle: { backgroundColor: colors.background.paper },
+              headerTintColor: colors.text.primary,
+              headerShadowVisible: false,
+              headerBackVisible: false,
+              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+            }}
+          />
+          <Stack.Screen
+            name="privacy"
+            options={{
+              headerShown: true,
+              title: '隐私政策',
+              headerStyle: { backgroundColor: colors.background.paper },
+              headerTintColor: colors.text.primary,
+              headerShadowVisible: false,
+              headerBackVisible: false,
+              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+            }}
+          />
           <Stack.Screen
             name="post/[postId]"
             options={{