feat(refactor): Implement 1A.1 Package Structure Refactoring

This commit implements the package structure refactoring as outlined in phase1.md (Task 1A.1).

Key changes include:
- Reorganized packages into `pkg/garmin` for public API and `internal/` for internal implementations.
- Updated all import paths to reflect the new structure.
- Consolidated types and client logic into their respective new packages.
- Updated `cmd/garth/main.go` to use the new public API.
- Fixed various compilation and test issues encountered during the refactoring process.
- Converted `internal/api/client/auth_test.go` to a functional test.

This establishes a solid foundation for future enhancements and improves maintainability.

internal/testutils/http.go

@@ -0,0 +1,14 @@
+package testutils
+
+import (
+	"net/http"
+	"net/http/httptest"
+)
+
+func MockJSONResponse(code int, body string) *httptest.Server {
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(code)
+		w.Write([]byte(body))
+	}))
+}

internal/testutils/mock_client.go

@@ -0,0 +1,24 @@
+package testutils
+
+import (
+	"errors"
+	"io"
+	"net/url"
+
+	"garmin-connect/internal/api/client"
+)
+
+// MockClient simulates API client for tests
+type MockClient struct {
+	RealClient *client.Client
+	FailEvery  int
+	counter    int
+}
+
+func (mc *MockClient) ConnectAPI(path string, method string, params url.Values, body io.Reader) ([]byte, error) {
+	mc.counter++
+	if mc.FailEvery != 0 && mc.counter%mc.FailEvery == 0 {
+		return nil, errors.New("simulated error")
+	}
+	return mc.RealClient.ConnectAPI(path, method, params, body)
+}