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>

gradle/libs.versions.toml

@@ -11,9 +11,9 @@ core-ktx = "1.18.0"
 activity-compose = "1.13.0"
 junit = "4.13.2"
 reorderable = "3.0.0"
-protobuf = "4.29.3"
-grpc = "1.69.0"
-grpcKotlin = "1.4.1"
+protobuf = "4.34.1"
+grpc = "1.80.0"
+grpcKotlin = "1.5.0"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "core-ktx" }