add Android app to README, CI workflow for APK builds

- README: add Fitness section with APK download link
- Dockerfile.android: containerized build with Rust, Android SDK/NDK,
  Java 21, Node 22, pnpm — builds and signs the APK
- CI workflow: builds APK in container on push, deploys to
  bocken.org/static/Bocken.apk via SCP

.gitea/workflows/android.yml

@@ -0,0 +1,38 @@
+name: Android APK
+
+on:
+  push:
+    branches: [ master ]
+    paths:
+      - 'src-tauri/**'
+      - 'src/**'
+      - 'static/**'
+      - 'package.json'
+      - 'pnpm-lock.yaml'
+      - 'Dockerfile.android'
+      - '.gitea/workflows/android.yml'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build APK in container
+        run: |
+          docker build -f Dockerfile.android -t bocken-android .
+          docker create --name apk-extract bocken-android
+          docker cp apk-extract:/tmp/Bocken.apk ./Bocken.apk
+          docker rm apk-extract
+
+      - name: Deploy APK to server
+        uses: appleboy/scp-action@master
+        with:
+          host: bocken.org
+          username: homepage
+          key: ${{ secrets.homepage_ssh }}
+          passphrase: ${{ secrets.homepage_pass }}
+          port: 22
+          source: "Bocken.apk"
+          target: "/var/www/static/"