Mobile: standalone photo demo + full-bleed full-screen cards

- New /demo flow (no login/backend): take/pick a photo → card front;
  swipe → back reveals QR + name/title. Built in-memory on device.
- CardFace renders image backgrounds (local URI or asset key) + borderRadius prop
- CardViewer `fullScreen` mode fills the device window (useWindowDimensions),
  radius 0, status bar hidden; applied to the demo and the real card viewer
- Landing screen offers the demo without signing in
- Added expo-image-picker (camera/photo perms); pinned expo-constants/expo-linking
  to SDK 51 (npm had resolved newer-SDK majors that broke the Gradle build)
- Native android/ is gitignored (regenerated by `expo prebuild`)

Built and deployed to a physical Android device (BUILD SUCCESSFUL).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-mobile/.gitignore

@@ -0,0 +1,18 @@
+# Expo / Metro
+.expo/
+dist/
+node_modules/
+
+# Native projects are generated by `expo prebuild` (CNG) — not committed.
+/android
+/ios
+
+# Misc
+*.log
+.DS_Store
+
+# @generated expo-cli sync-2b81b286409207a5da26e14c78851eb30d8ccbdb
+# The following patterns were generated by expo-cli
+
+expo-env.d.ts
+# @end expo-cli