feat: add UMS CLI for binary packaging and system initialization

- Add Cobra-based CLI with ums init, ums serve, ums version commands
- ums init supports interactive prompts and non-interactive flags
- Generates secure JWT secrets and config.yaml automatically
- Extract server.Serve() function for reuse
- Add cross-platform build targets to Makefile
- Update README with CLI installation and usage instructions

New files:
- cmd/ums/main.go - CLI entry point
- cmd/ums/cmd/root.go - Root command
- cmd/ums/cmd/init.go - Interactive/non-interactive init
- cmd/ums/cmd/serve.go - Server command
- cmd/ums/cmd/version.go - Version command
- internal/server/server.go - Extracted Serve function

cmd/ums/main.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"github.com/user-management-system/cmd/ums/cmd"
+)
+
+func main() {
+	cmd.Execute()
+}