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.

pkg/garmin/doc.go

@@ -0,0 +1,46 @@
+// Package garth provides a comprehensive Go client for the Garmin Connect API.
+// It offers full coverage of Garmin's health and fitness data endpoints with
+// improved performance and type safety over the original Python implementation.
+//
+// Key Features:
+// - Complete implementation of Garmin Connect API (data and stats endpoints)
+// - Automatic session management and token refresh
+// - Concurrent data retrieval with configurable worker pools
+// - Comprehensive error handling with detailed error types
+// - 3-5x performance improvement over Python implementation
+//
+// Usage:
+//
+//	client, err := garth.NewClient("garmin.com")
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//
+//	err = client.Login("email", "password")
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//
+//	// Get yesterday's body battery data
+//	bb, err := garth.BodyBatteryData{}.Get(time.Now().AddDate(0,0,-1), client)
+//
+//	// Get weekly steps
+//	steps := garth.NewDailySteps()
+//	stepData, err := steps.List(time.Now(), 7, client)
+//
+// Error Handling:
+// The package defines several error types that implement the GarthError interface:
+//   - APIError: HTTP/API failures (includes status code and response body)
+//   - IOError: File/network issues
+//   - AuthError: Authentication failures
+//   - OAuthError: Token management issues
+//   - ValidationError: Input validation failures
+//
+// Performance:
+// Benchmarks show significant performance improvements over Python:
+//   - BodyBattery Get: 1195x faster
+//   - Sleep Data Get: 1190x faster
+//   - Steps List (7 days): 1216x faster
+//
+// See README.md for additional usage examples and CLI tool documentation.
+package garmin