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>

Makefile

@@ -5,7 +5,11 @@ AVD          ?= DC-1
 PACKAGE      := net.metacircular.engpad
 ACTIVITY     := $(PACKAGE).MainActivity
 
-.PHONY: build test lint clean run devrun apk all
+# Proto generation output directory (checked into git)
+PROTO_GEN    := app/src/main/java/gen
+PROTO_SRC    := app/src/main/proto
+
+.PHONY: build test lint clean run devrun apk proto proto-lint all
 
 # Build everything (compile + test + lint)
 all: lint test build
@@ -31,6 +35,26 @@ apk:
 	./gradlew assembleRelease
 	@echo "APK: app/build/outputs/apk/release/app-release-unsigned.apk"
 
+# Generate Kotlin/gRPC stubs from .proto files
+proto:
+	@rm -rf $(PROTO_GEN)
+	@mkdir -p $(PROTO_GEN)
+	protoc \
+		--proto_path=$(PROTO_SRC) \
+		--proto_path=$$(protoc --print-proto-path 2>/dev/null || echo /usr/local/include) \
+		--java_out=lite:$(PROTO_GEN) \
+		--kotlin_out=lite:$(PROTO_GEN) \
+		--plugin=protoc-gen-grpc=$$(which protoc-gen-grpc-java 2>/dev/null || echo $(HOME)/bin/protoc-gen-grpc-java) \
+		--grpc_out=lite:$(PROTO_GEN) \
+		--plugin=protoc-gen-grpckt=$(HOME)/bin/protoc-gen-grpckt \
+		--grpckt_out=lite:$(PROTO_GEN) \
+		$$(find $(PROTO_SRC) -name '*.proto')
+
+# Lint proto files with buf
+proto-lint:
+	buf lint
+	buf breaking --against '.git#branch=master,subdir=$(PROTO_SRC)'
+
 # Clean build artifacts
 clean:
 	./gradlew clean