feat: switch iperf3 to JNI with 3.19 source

- Switch from bundled iperf3 binary (Process.start) to JNI compilation
- iperf3 3.21 source API was incompatible (start_time, iperf_strerror, etc.)
- iperf3 3.19 source is proven to work (used by CellularLab)
- CMakeLists.txt updated: config.h generated into cpp/iperf/ for correct include paths
- iperf_jni.c fixed for 3.19 API (no start_time on iperf_test)
- getTestProgress() returns g_test->duration as fallback (no real-time elapsed tracking yet)
- NDK CMake pinned to 3.22.1
- UI layer updated: removed Iperf3BinaryPathProvider references
- Tests updated: TestRunViewModelTest matches new service interface
- pubspec.yaml: removed iperf3 assets bundle, added http dependency
- AndroidManifest: INTERNET permission (needed for hostname resolution)

android/app/src/main/cpp/iperf/iperf_pthread.h

@@ -0,0 +1,21 @@
+#include "iperf_config.h"
+
+#if defined(HAVE_PTHREAD)
+
+#include <pthread.h>
+
+#if defined(__ANDROID__)
+
+/* Adding missing `pthread` related definitions in Android.
+ */
+
+#define PTHREAD_CANCEL_ASYNCHRONOUS 0
+#define PTHREAD_CANCEL_ENABLE 0
+
+int pthread_setcanceltype(int type, int *oldtype);
+int pthread_setcancelstate(int state, int *oldstate);
+int pthread_cancel(pthread_t thread_id);
+
+#endif // defined(__ANDROID__)
+
+#endif // defined(HAVE_PTHREAD)