feat: add 4Hz GPS update interval and CI workflow

- Set intervalDuration: 250ms in AndroidSettings for 4Hz updates.
- Add CI workflow (analyze + release APK) modelled on iperf3-gui.

.github/workflows/ci.yaml

@@ -0,0 +1,41 @@
+name: CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
+
+# Target the self-hosted Flutter/Android runner at the android-stuff org.
+# See tool/setup_ci_runner.sh for how that runner is provisioned.
+#
+# The runner is host-mode so the same JDK 17 + Android SDK + Flutter
+# toolchain is used as the verified release — nothing installs per-run.
+
+jobs:
+  analyze:
+    name: Analyze & Build
+    runs-on: flutter-android
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Toolchain
+        run: |
+          flutter --version
+          echo "ANDROID_HOME=${ANDROID_HOME:?not set on this runner}"
+
+      - name: Get dependencies
+        run: flutter pub get
+
+      - name: Analyze
+        run: flutter analyze
+
+      - name: Build release APK
+        run: flutter build apk --release
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: app-release-apk
+          path: build/app/outputs/flutter-apk/app-release.apk
+          if-no-files-found: error

lib/data/services/gps_service.dart

@@ -15,9 +15,10 @@ class GpsService {
     LocationAccuracy accuracy = LocationAccuracy.high,
   }) {
     return Geolocator.getPositionStream(
-      locationSettings: LocationSettings(
+      locationSettings: AndroidSettings(
         accuracy: accuracy,
         distanceFilter: 0,
+        intervalDuration: Duration(milliseconds: 250),
       ),
     );
   }