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/app/card/[cardId]/view.tsx

@@ -41,7 +41,7 @@ export default function CardViewScreen() {
         </View>
       ) : (
         <>
-          <CardViewer card={data.definition} />
+          <CardViewer card={data.definition} fullScreen />
           <ShareSheet
             cardId={cardId}
             handle={data.handle}