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/data/body_battery_test.go

@@ -0,0 +1,122 @@
+package data
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseBodyBatteryReadings(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    [][]any
+		expected []BodyBatteryReading
+	}{
+		{
+			name: "valid readings",
+			input: [][]any{
+				{1000, "ACTIVE", 75, 1.0},
+				{2000, "ACTIVE", 70, 1.0},
+				{3000, "REST", 65, 1.0},
+			},
+			expected: []BodyBatteryReading{
+				{1000, "ACTIVE", 75, 1.0},
+				{2000, "ACTIVE", 70, 1.0},
+				{3000, "REST", 65, 1.0},
+			},
+		},
+		{
+			name: "invalid readings",
+			input: [][]any{
+				{1000, "ACTIVE", 75},           // missing version
+				{2000, "ACTIVE"},               // missing level and version
+				{3000},                         // only timestamp
+				{"invalid", "ACTIVE", 75, 1.0}, // wrong timestamp type
+			},
+			expected: []BodyBatteryReading{},
+		},
+		{
+			name:     "empty input",
+			input:    [][]any{},
+			expected: []BodyBatteryReading{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := ParseBodyBatteryReadings(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestParseStressReadings(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    [][]int
+		expected []StressReading
+	}{
+		{
+			name: "valid readings",
+			input: [][]int{
+				{1000, 25},
+				{2000, 30},
+				{3000, 20},
+			},
+			expected: []StressReading{
+				{1000, 25},
+				{2000, 30},
+				{3000, 20},
+			},
+		},
+		{
+			name: "invalid readings",
+			input: [][]int{
+				{1000},        // missing stress level
+				{2000, 30, 1}, // extra value
+				{},            // empty
+			},
+			expected: []StressReading{},
+		},
+		{
+			name:     "empty input",
+			input:    [][]int{},
+			expected: []StressReading{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := ParseStressReadings(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestDailyBodyBatteryStress(t *testing.T) {
+	now := time.Now()
+	d := DailyBodyBatteryStress{
+		CalendarDate: now,
+		BodyBatteryValuesArray: [][]any{
+			{1000, "ACTIVE", 75, 1.0},
+			{2000, "ACTIVE", 70, 1.0},
+		},
+		StressValuesArray: [][]int{
+			{1000, 25},
+			{2000, 30},
+		},
+	}
+
+	t.Run("body battery readings", func(t *testing.T) {
+		readings := ParseBodyBatteryReadings(d.BodyBatteryValuesArray)
+		assert.Len(t, readings, 2)
+		assert.Equal(t, 75, readings[0].Level)
+	})
+
+	t.Run("stress readings", func(t *testing.T) {
+		readings := ParseStressReadings(d.StressValuesArray)
+		assert.Len(t, readings, 2)
+		assert.Equal(t, 25, readings[0].StressLevel)
+	})
+}