fix(windows): don't crash on startup from OCR apartment

The app showed no window because RegisterOcrChannel ran
winrt::init_apartment() (defaults to MTA) on the Flutter platform
thread, which main.cpp already put in an STA via CoInitializeEx. That
throws RPC_E_CHANGED_MODE and kills the app before the window appears.
Remove that call; run the WinRT OCR on a dedicated MTA worker thread
per recognize call instead, joining before returning the result.

windows/runner/ocr_channel.cpp

@@ -7,6 +7,7 @@
 
 #include <memory>
 #include <string>
+#include <thread>
 #include <vector>
 
 #include <winrt/Windows.Foundation.h>
@@ -70,8 +71,12 @@ std::unique_ptr<flutter::MethodChannel<flutter::EncodableValue>> g_ocr_channel;
 }  // namespace
 
 void RegisterOcrChannel(flutter::FlutterEngine* engine) {
-  winrt::init_apartment();
-
+  // NOTE: do NOT call winrt::init_apartment() here. This runs on the Flutter
+  // platform thread, which main.cpp has already initialized as an STA via
+  // CoInitializeEx(COINIT_APARTMENTTHREADED). Calling init_apartment() (which
+  // defaults to multi-threaded) would throw RPC_E_CHANGED_MODE and crash the
+  // app before the window appears. The WinRT OCR work runs on its own
+  // dedicated MTA worker thread per call instead (see below).
   g_ocr_channel =
       std::make_unique<flutter::MethodChannel<flutter::EncodableValue>>(
           engine->messenger(), "badnote/ocr",
@@ -91,14 +96,31 @@ void RegisterOcrChannel(flutter::FlutterEngine* engine) {
           return;
         }
 
-        try {
-          const auto png_bytes = ExtractBytes(*call.arguments());
-          const auto text = RecognizePng(png_bytes);
+        const auto png_bytes = ExtractBytes(*call.arguments());
+
+        // Run the WinRT OCR on a dedicated multi-threaded-apartment worker so
+        // its blocking .get() calls behave correctly and the platform thread's
+        // STA apartment is left untouched. We join before returning so the
+        // MethodResult is completed on the platform thread.
+        std::string text;
+        std::string error;
+        std::thread worker([&]() {
+          try {
+            winrt::init_apartment(winrt::apartment_type::multi_threaded);
+            text = RecognizePng(png_bytes);
+            winrt::uninit_apartment();
+          } catch (const winrt::hresult_error& e) {
+            error = winrt::to_string(e.message());
+          } catch (...) {
+            error = "Unknown OCR error";
+          }
+        });
+        worker.join();
+
+        if (!error.empty()) {
+          result->Error("ocr_failed", error);
+        } else {
           result->Success(flutter::EncodableValue(text));
-        } catch (const winrt::hresult_error& error) {
-          result->Error("ocr_failed", winrt::to_string(error.message()));
-        } catch (...) {
-          result->Error("ocr_failed", "Unknown OCR error");
         }
       });
 }