feat: add iOS and Android screenshot capture guides (FRE-4572)

packages/mobile/assets/store/ios/screenshots/README.md

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+# iOS Screenshots
+
+Place screenshots in this directory before submission.
+
+## Required Sizes
+- `iphone-6.7in.png` - 1290x2796 (iPhone 14 Pro Max, 15 Pro Max)
+- `iphone-6.1in.png` - 1170x2532 (iPhone 14, 15)
+
+## Screens to Capture
+1. Dashboard - exposure summary cards
+2. DarkWatch - watch list with entities
+3. SpamShield - call/text history
+4. VoicePrint - family member enrollment
+5. Settings - tier and notifications
+
+## Notes
+- Use Xcode simulator or `xcrun simctl io booted screenshot` for captures
+- No device frame overlays in screenshots
+- Ensure dark mode and light mode are represented