Add Docker-based Android APK build tooling

Multi-stage Dockerfile.android (Bun + Android SDK + Gradle) built via
build-apk.sh, so the APK can be built reproducibly without a local
Android SDK install.

build-apk.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+set -e
+
+if command -v podman > /dev/null 2>&1; then
+    DOCKER=podman
+elif command -v docker > /dev/null 2>&1; then
+    DOCKER=docker
+else
+    echo "Fehler: weder podman noch docker gefunden." >&2
+    exit 1
+fi
+
+OUT="$(pwd)/apk-output"
+mkdir -p "$OUT"
+
+# Immer ohne Cache bauen – verhindert dass alte Schichten das Ergebnis verfälschen
+$DOCKER build --no-cache \
+    -f Dockerfile.android \
+    --output "type=local,dest=$OUT" \
+    .
+
+echo ""
+echo "✓ APK bereit: $OUT/tiptoi-sync.apk"