feat(internal): add platform detection and auth packages

- internal/platform: cross-platform path detection, 6 tests
- internal/auth: JWT generation/validation + bcrypt, 8 tests
- 89 total tests, build + vet clean

backend/internal/platform/platform_test.go

@@ -0,0 +1,64 @@
+package platform
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestDetect(t *testing.T) {
+	p := Detect()
+
+	if p.OS != runtime.GOOS {
+		t.Errorf("expected OS=%s, got %s", runtime.GOOS, p.OS)
+	}
+	if p.Arch != runtime.GOARCH {
+		t.Errorf("expected Arch=%s, got %s", runtime.GOARCH, p.Arch)
+	}
+	if runtime.GOOS == "linux" && !p.IsLinux {
+		t.Error("expected IsLinux=true on Linux")
+	}
+}
+
+func TestDataDir(t *testing.T) {
+	p := Detect()
+	dir := p.DataDir()
+	if dir == "" {
+		t.Error("DataDir should not be empty")
+	}
+}
+
+func TestConfigDir(t *testing.T) {
+	p := Detect()
+	dir := p.ConfigDir()
+	if dir == "" {
+		t.Error("ConfigDir should not be empty")
+	}
+}
+
+func TestCacheDir(t *testing.T) {
+	p := Detect()
+	dir := p.CacheDir()
+	if dir == "" {
+		t.Error("CacheDir should not be empty")
+	}
+}
+
+func TestSummary(t *testing.T) {
+	p := Detect()
+	s := p.Summary()
+	if s == "" {
+		t.Error("Summary should not be empty")
+	}
+	// Should contain the OS name
+	if len(s) < 3 {
+		t.Errorf("Summary too short: %s", s)
+	}
+}
+
+func TestDefaultNATSURL(t *testing.T) {
+	p := Detect()
+	url := p.DefaultNATSURL()
+	if url != "nats://localhost:4222" {
+		t.Errorf("expected nats://localhost:4222, got %s", url)
+	}
+}