OCR: embedded, cross-platform ONNX backend with pluggable fallback

Make on-device OCR a pluggable local service so it runs locally on every
platform (not just Windows), aimed at GoodNotes/Notability-class handwriting on
low-power hardware (e.g. Zen2 APU, CPU/iGPU).

- New OcrBackend abstraction (lib/services/ocr/): selector prefers an embedded
  ONNX recognition backend, falling back to the OS-native backend (Windows
  WinRT), and to a clean no-op when neither is available.
- OnnxRecognitionBackend: flutter_onnxruntime session from a bundled asset,
  dart:ui preprocessing (resize to 48px, CHW float32, normalized), pure-Dart CTC
  greedy decode. Fully guarded — absent model/dict is a no-op; never throws.
- ocr_engine.dart kept as a thin facade (recognizeImage) delegating to the
  selector, so ocr_service.dart is unchanged.
- CtcDecoder unit-tested (6 tests). flutter analyze clean; all tests pass.
- Model is not committed; tool/fetch_ocr_model.sh + assets/models/ocr/README.md
  document fetching PP-OCRv4 rec + dict on the dev machine.
- CI: forward HTTPS_PROXY to the Windows build so CMake can fetch the ONNX
  Runtime native lib behind the GFW; README documents the system-install
  alternative. PP-OCR geometry/blank assumptions documented for on-device tuning.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -48,7 +48,16 @@ jobs:
       - name: Enable Windows desktop
         run: flutter config --enable-windows-desktop
 
+      # The flutter_onnxruntime plugin's CMake downloads the ONNX Runtime native
+      # library from github.com/microsoft/onnxruntime/releases at build time.
+      # That host is blocked here, but CMake's file(DOWNLOAD) honours proxy env
+      # vars, so we forward HTTP(S)_PROXY (set them as repo secrets, e.g.
+      # http://127.0.0.1:7890). Alternatively install ONNX Runtime system-wide
+      # and pass -DUSE_SYSTEM_ONNXRUNTIME=ON -DONNXRUNTIME_ROOT_DIR=... .
       - name: Build Windows release
+        env:
+          HTTP_PROXY: ${{ secrets.HTTP_PROXY }}
+          HTTPS_PROXY: ${{ secrets.HTTPS_PROXY }}
         run: flutter build windows --release
 
       - name: Package artifact