android: add BAL Reader app (Chaquopy codec bridge) + frame progress bar

First commit of the Android 'BAL Reader' app: a camera scanner that decodes
the same four transfer formats the plugin exports (BAL QR v2/legacy, BC-UR
v1/v2, BBQR) by running the synced bal/core codecs verbatim under Chaquopy
plus a verify_chain regression. Adds android/scripts/build_apk.py (resync
codecs, gradle build, prints APK sha256) and sync_codecs.py.

UI: during capture a thin mint fill-bar above the camera preview shows
received/total frames (FrameProgressBar), complementing the exact n/N label.

android/app/proguard-rules.pro

@@ -0,0 +1,5 @@
+# Chaquopy Python runtime.
+-keep class com.chaquo.python.** { *; }
+
+# ML Kit barcode scanning.
+-keep class com.google.mlkit.** { *; }