feat(rn-client-demo): add React Native client demo example

Expo/React Native app demonstrating protoc-gen-ts-client with
TaskService CRUD, validation errors, headers, abort, and unwrap.
Uses expo/AppEntry as the correct Expo entry point.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: SebastienMelki

examples/rn-client-demo/.gitignore

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+# Generated files
+api/
+app/generated/
+
+# Node.js
+app/node_modules/
+app/.expo/
+
+# Buf lock file (will be created on first buf dep update)
+buf.lock
+
+# Go sum file (will be created on first go mod tidy)
+go.sum

examples/rn-client-demo/Makefile

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+.PHONY: demo generate server install-app start-app clean build-plugins
+
+# Full demo: generate code, start server, install app deps
+demo: generate install-app
+	@echo ""
+	@echo "Starting Go server in background..."
+	@go run main.go & SERVER_PID=$$!; \
+	echo "Waiting for server to be ready..."; \
+	for i in $$(seq 1 30); do \
+		if curl -s -o /dev/null http://localhost:3000/api/v1/tasks 2>/dev/null; then \
+			break; \
+		fi; \
+		sleep 1; \
+	done; \
+	echo ""; \
+	echo "Server is ready! Start the Expo app with:"; \
+	echo "  cd app && npx expo start"; \
+	echo ""; \
+	echo "Press Ctrl+C to stop the server."; \
+	wait $$SERVER_PID
+
+# Build the protoc plugin binaries
+build-plugins:
+	@echo "Building protoc plugins..."
+	@cd ../.. && go build -o bin/protoc-gen-go-http ./cmd/protoc-gen-go-http
+	@cd ../.. && go build -o bin/protoc-gen-ts-client ./cmd/protoc-gen-ts-client
+
+# Generate Go + TypeScript code from proto
+generate: build-plugins
+	@echo "Fetching buf dependencies..."
+	@buf dep update
+	@echo "Generating code..."
+	@buf generate
+	@echo "Updating Go modules..."
+	@go mod tidy
+	@echo "Code generated successfully"
+
+# Run the Go server (blocks)
+server: generate
+	@go run main.go
+
+# Install Expo app dependencies
+install-app:
+	@echo "Installing Expo app dependencies..."
+	@cd app && npm install --silent
+	@cd app && npx expo install --fix
+
+# Start Expo dev server (assumes npm install done)
+start-app:
+	@cd app && npx expo start
+
+# Clean generated files
+clean:
+	@rm -rf api/ app/generated/ app/node_modules/ app/.expo/
+	@echo "Cleaned generated files"