Add live edge-detection overlay while aiming; v0.2.0

Bind a CameraX ImageAnalysis use case that runs EdgeDetector on downscaled
YUV frames (throttled ~6/s) and draw the detected document quad as a live
green overlay on the preview, mapped through the FILL_CENTER crop. Preview
switched to TextureView (COMPATIBLE) mode so the Compose overlay renders on
top. Bump versionCode 4 / versionName 0.2.0.

Fixes #4

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/build.gradle.kts

@@ -24,8 +24,8 @@ android {
         applicationId = "eu.geyskens.pdfscan"
         minSdk = 26
         targetSdk = 35
-        versionCode = 3
-        versionName = "0.1.2"
+        versionCode = 4
+        versionName = "0.2.0"
 
         // OpenCV native libs are large; ship only the ABIs real phones use.
         ndk {