fix: address code review findings for mobile app (FRE-4572)

P0 fixes:
- Replace crypto.randomUUID() with uuid v4 (not available in RN)
- Replace Platform.Version with expo-device osVersion
- Fix auth navigation types, remove unused App route

P1 fixes:
- Push notification handler respects user preferences (useRef pattern)
- Fix stale closure: use zustand subscribe + useRef for live preferences
- Add retry logging for device registration failures
- Replace emoji tab icons with @expo/vector-icons Ionicons
- Document API integration TODOs in all local-only stores

P2 fixes:
- Add __DEV__ global declaration (global.d.ts)
- Fix package.json main field to expo/AppEntry.js
- Add retry logging for push device registration
- Add z-index/elevation to LoadingOverlay
- Add visual indicator to EmptyState icon

P3 fixes:
- Type navigation with NavigationProp<RootStackParamList>
- Move getSeverityColor to theme.ts (single source of truth)
- Add useMemo for SpamShield filter computations
- Verified usesNonExemptEncryption: false is correct for expo-secure-store

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/mobile/src/constants/theme.ts

@@ -52,6 +52,15 @@ export const SHADOWS = {
 
 export const API_URL = process.env.EXPO_PUBLIC_API_URL || 'https://api.shieldai.freno.me/api/v1';
 
+export const getSeverityColor = (severity: string): string => {
+  switch (severity) {
+    case 'critical': return COLORS.danger;
+    case 'high': return COLORS.warning;
+    case 'medium': return COLORS.accent;
+    default: return COLORS.textSecondary;
+  }
+};
+
 export const TIER_FEATURES = {
   free: {
     name: 'Free',