Fix protobuf setup: canonical proto, buf lint, Makefile proto target

- Restored canonical proto from eng-pad-server (PageData, StrokeData
  message names, not Notebook/Page/Stroke)
- Added java_package + java_multiple_files options
- Renamed service to EngPadSyncService (buf STANDARD lint compliance)
- Simplified build.gradle.kts: removed broken custom GenerateProtoTask,
  proto stubs generated via Makefile `make proto` and checked into git
- Generated stubs in app/src/main/java/gen/
- Fixed SyncClient/SyncManager to match canonical proto schema
- Updated dependency versions: protobuf 4.34.1, grpc 1.80.0, grpcKotlin 1.5.0
- Added buf.yaml with STANDARD lint rules and FILE breaking detection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

buf.yaml

@@ -0,0 +1,12 @@
+version: v2
+
+modules:
+  - path: app/src/main/proto
+
+lint:
+  use:
+    - STANDARD
+
+breaking:
+  use:
+    - FILE