Verify builds on both platforms; pin Android SDK levels

compileSdk 37 (a dependency requires it; the build fails on 36), minSdk 26 for
parity with the native app, targetSdk 36. AGP auto-installed platform 37, CMake
and a 2.8GB NDK during the build since licences were already accepted.

Both artifacts confirmed: Runner.app and app-debug.apk.

Recorded two deprecation warnings against flutter_foreground_task, the plugin
chosen for Android background liveness in T12: no Swift Package Manager support
on iOS, and it applies KGP on Android. Both are future hard errors, so the
LocationSource seam in T10 matters more than assumed.

Also recorded the disk episode: the plan braced for SDK installs, but builds
were what filled the disk (22 GiB to 3.9 GiB). Recovered to 11 GiB.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -46,3 +46,7 @@ app.*.map.json
 
 # Widget Preview related
 .widget_preview/
+
+# Build artifacts (large; regenerable)
+build/
+.dart_tool/