test(backend): add unit and integration tests for folder browsing

- Add unit tests in internal/services/library_service_test.go
  - Test path traversal protection
  - Test non-existent path handling
  - Test file vs directory validation
  - Test successful directory listing
- Add integration tests in cmd/server/tests/library_browse_test.go
  - Use setupTestServer() helper from test_helpers.go
  - Test no authentication returns 401
  - Test regular user returns 403 forbidden
  - Test admin can browse directories
  - Test path traversal blocking
- All tests use table-driven approach with t.Run()

Fixes: Issue 2 (tests)

cmd/server/tests/library_browse_test.go

@@ -0,0 +1,62 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBrowseLibraryFoldersEndpoint(t *testing.T) {
+	setup := setupTestServer(t)
+	defer setup.Close()
+
+	t.Run("GET /api/libraries/browse - no authentication returns 401", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/api/libraries/browse?path=/tmp", nil)
+		rec := httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		assert.Equal(t, http.StatusUnauthorized, rec.Code)
+	})
+
+	t.Run("GET /api/libraries/browse - regular user returns 403 forbidden", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/api/libraries/browse?path=/tmp", nil)
+		req.Header.Set("Authorization", "Bearer "+setup.RegularToken)
+		rec := httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		assert.Equal(t, http.StatusForbidden, rec.Code)
+	})
+
+	t.Run("GET /api/libraries/browse - admin can browse directories", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/api/libraries/browse?path=/tmp", nil)
+		req.Header.Set("Authorization", "Bearer "+setup.Token)
+		rec := httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		assert.Equal(t, http.StatusOK, rec.Code)
+
+		var data struct {
+			CurrentPath string   `json:"current_path"`
+			ParentPath  string   `json:"parent_path"`
+			Directories []string `json:"directories"`
+		}
+		err := json.Unmarshal(rec.Body.Bytes(), &data)
+		assert.NoError(t, err)
+		assert.Equal(t, "/tmp", data.CurrentPath)
+		assert.NotEmpty(t, data.Directories)
+	})
+
+	t.Run("GET /api/libraries/browse - blocks path traversal attempts", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/api/libraries/browse?path=/etc/../root", nil)
+		req.Header.Set("Authorization", "Bearer "+setup.Token)
+		rec := httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		assert.Equal(t, http.StatusBadRequest, rec.Code)
+
+		var errResp struct {
+			Error string `json:"error"`
+		}
+		json.Unmarshal(rec.Body.Bytes(), &errResp)
+		assert.Contains(t, errResp.Error, "path traversal not allowed")
+	})
+}