feat: add version flag and Makefile for release builds

- Add -version flag to show build version
- Add Makefile with build targets:
  - make build-release: optimized build with version
  - make build-all: cross-compile for Linux/macOS
  - make run/test/clean: dev commands
- Update documentation with build commands
- Version is embedded via ldflags during build

Usage:
  make build-release  # Build with version
  ./bin/qfs -version  # Show version

Version format: v0.2.5-1-gfa0f5e3 (tag-commits-hash)

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

cmd/qfs/main.go

@@ -34,11 +34,21 @@ const (
 	localDBPath = "./data/settings.db"
 )
 
+// Version is set via ldflags during build
+var Version = "dev"
+
 func main() {
 	configPath := flag.String("config", "config.yaml", "path to config file (optional, for server settings)")
 	migrate := flag.Bool("migrate", false, "run database migrations")
+	version := flag.Bool("version", false, "show version information")
 	flag.Parse()
 
+	// Show version if requested
+	if *version {
+		fmt.Printf("qfs version %s\n", Version)
+		os.Exit(0)
+	}
+
 	// Initialize local SQLite database (always used)
 	local, err := localdb.New(localDBPath)
 	if err != nil {