feat: add Windows support to build system

- Add make build-windows for Windows AMD64
- Update make build-all to include Windows
- Update release script to package Windows binary as .zip
- Add Windows installation instructions to docs
- Windows binary: qfs-windows-amd64.exe (~17MB)

All platforms now supported:
- Linux AMD64 (.tar.gz)
- macOS Intel/ARM (.tar.gz)
- Windows AMD64 (.zip)

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

Makefile

@@ -33,8 +33,14 @@ build-macos:
 	@echo "✓ Built: bin/$(BINARY)-darwin-amd64"
 	@echo "✓ Built: bin/$(BINARY)-darwin-arm64"
 
+# Build release for Windows (cross-compile)
+build-windows:
+	@echo "Building $(BINARY) for Windows..."
+	CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build -ldflags="$(LDFLAGS)" -o bin/$(BINARY)-windows-amd64.exe ./cmd/qfs
+	@echo "✓ Built: bin/$(BINARY)-windows-amd64.exe"
+
 # Build all platforms
-build-all: build-release build-linux build-macos
+build-all: build-release build-linux build-macos build-windows
 
 # Create release packages for all platforms
 release:
@@ -77,6 +83,7 @@ help:
 	@echo "  build-release  Build optimized release (default)"
 	@echo "  build-linux    Cross-compile for Linux"
 	@echo "  build-macos    Cross-compile for macOS (Intel + Apple Silicon)"
+	@echo "  build-windows  Cross-compile for Windows"
 	@echo "  build-all      Build for all platforms"
 	@echo "  release        Create release packages for all platforms"
 	@echo "  version        Show current version"