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>

lib/services/ocr/ocr_backend.dart

@@ -0,0 +1,19 @@
+import 'dart:typed_data';
+
+/// A pluggable local OCR backend.
+///
+/// Implementations turn a PNG image into recognized text. The app selects an
+/// available backend via [OcrBackends]; absence of any backend is a clean
+/// no-op (recognition returns null).
+abstract class OcrBackend {
+  /// Short identifier used for logging/selection (e.g. 'native', 'onnx').
+  String get name;
+
+  /// Whether this backend can run on the current device/build. May perform a
+  /// lazy initialization attempt (e.g. loading a model) the first time.
+  Future<bool> isAvailable();
+
+  /// Recognize text from a PNG image. Returns null when nothing is recognized
+  /// or the backend is unavailable. Implementations must never throw.
+  Future<String?> recognize(Uint8List pngBytes);
+}