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.

Dockerfile.android

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+# ── Stage 1: Tools (wird gecacht – nur bei SDK-Änderungen neu gebaut) ────────
+FROM eclipse-temurin:21-jdk-jammy AS tools
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        curl unzip wget ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Bun
+RUN curl -fsSL https://bun.sh/install | bash
+ENV PATH="/root/.bun/bin:$PATH"
+# Symlink damit Tools mit #!/usr/bin/env node funktionieren
+RUN ln -s /root/.bun/bin/bun /usr/local/bin/node
+
+# Android SDK
+ENV ANDROID_HOME=/opt/android-sdk
+ENV PATH="$PATH:$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools"
+
+RUN mkdir -p $ANDROID_HOME/cmdline-tools && \
+    wget -q https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip \
+         -O /tmp/tools.zip && \
+    unzip -q /tmp/tools.zip -d $ANDROID_HOME/cmdline-tools && \
+    mv $ANDROID_HOME/cmdline-tools/cmdline-tools $ANDROID_HOME/cmdline-tools/latest && \
+    rm /tmp/tools.zip
+
+RUN yes | sdkmanager --licenses > /dev/null 2>&1 && \
+    sdkmanager \
+        "platforms;android-35" \
+        "build-tools;35.0.0" \
+        "platform-tools"
+
+# ── Stage 2: Build ────────────────────────────────────────────────────────────
+FROM tools AS builder
+
+WORKDIR /app
+
+# Abhängigkeiten cachen
+COPY client/package.json client/bun.lock* ./client/
+RUN cd client && bun install
+
+COPY . .
+
+# 1. Web-App bauen (ohne PWA Service Worker für Capacitor)
+RUN cd client && CAPACITOR=1 bun run build
+
+# 2. Web-Assets in das Android-Projekt synchronisieren
+#    (android/ ist im Repo eingecheckt – kein cap add nötig)
+RUN cd client && CI=true ./node_modules/.bin/cap sync android
+
+# 3. APK bauen
+RUN cd client/android && \
+    chmod +x gradlew && \
+    GRADLE_OPTS="-Xmx3g -XX:MaxMetaspaceSize=512m" \
+    ./gradlew assembleDebug --no-daemon
+
+# ── Stage 3: Nur die APK (für --output) ──────────────────────────────────────
+FROM scratch
+COPY --from=builder \
+    /app/client/android/app/build/outputs/apk/debug/app-debug.apk \
+    /tiptoi-sync.apk

build-apk.sh

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+#!/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"