feat: Add start-to-current weight delta in stats display (#3)

Display weight change from first entry to latest in the stats header.
Shows "Lost X lbs/kg", "Gained X lbs/kg", or "No change" based on delta.
Includes comprehensive test coverage for all scenarios.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: tryonlinux

internal/display/table.go

@@ -49,13 +49,28 @@ func RenderWeightsTable(weights []models.Weight, settings *models.Settings, limi
 	avgWeight := totalWeight / float64(len(weights))
 	latestWeight := weights[0].Weight
 	latestBMI := weights[0].BMI
+	startWeight := weights[len(weights)-1].Weight
+
+	// Calculate change from start
+	delta := latestWeight - startWeight
+	var deltaStr string
+	if delta < 0 {
+		// Lost weight
+		deltaStr = fmt.Sprintf("Lost %s", FormatWeight(math.Abs(delta), settings.WeightUnit))
+	} else if delta > 0 {
+		// Gained weight
+		deltaStr = fmt.Sprintf("Gained %s", FormatWeight(delta, settings.WeightUnit))
+	} else {
+		deltaStr = "No change"
+	}
 
 	// Build stats header (goes with ASCII art header)
 	var header strings.Builder
-	header.WriteString(HeaderStyle.Render(fmt.Sprintf("Latest: %s | BMI: %s | Avg: %s",
+	header.WriteString(HeaderStyle.Render(fmt.Sprintf("Latest: %s | BMI: %s | Avg: %s | %s",
 		FormatWeight(latestWeight, settings.WeightUnit),
 		FormatBMI(latestBMI),
-		FormatWeight(avgWeight, settings.WeightUnit))))
+		FormatWeight(avgWeight, settings.WeightUnit),
+		deltaStr)))
 	header.WriteString("\n")
 	header.WriteString(InfoStyle.Render(fmt.Sprintf("Min: %s | Max: %s | Entries: %d",
 		FormatWeight(minWeight, settings.WeightUnit),

tests/display_test.go

@@ -1,9 +1,11 @@
 package tests
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/tryonlinux/thicc/internal/display"
+	"github.com/tryonlinux/thicc/internal/models"
 )
 
 func TestFormatWeight(t *testing.T) {
@@ -60,3 +62,80 @@ func TestFormatDate(t *testing.T) {
 		}
 	}
 }
+
+func TestRenderWeightsTableDelta(t *testing.T) {
+	tests := []struct {
+		name        string
+		weights     []models.Weight
+		settings    *models.Settings
+		expectedStr string
+	}{
+		{
+			name: "weight loss shows 'Lost'",
+			weights: []models.Weight{
+				{ID: 3, Date: "2024-01-15", Weight: 150.0, BMI: 22.0},
+				{ID: 2, Date: "2024-01-08", Weight: 155.0, BMI: 22.5},
+				{ID: 1, Date: "2024-01-01", Weight: 160.0, BMI: 23.0},
+			},
+			settings: &models.Settings{
+				WeightUnit: "lbs",
+				HeightUnit: "in",
+				Height:     70,
+				GoalWeight: 145,
+			},
+			expectedStr: "Lost 10.00 lbs",
+		},
+		{
+			name: "weight gain shows 'Gained'",
+			weights: []models.Weight{
+				{ID: 3, Date: "2024-01-15", Weight: 165.0, BMI: 23.5},
+				{ID: 2, Date: "2024-01-08", Weight: 160.0, BMI: 23.0},
+				{ID: 1, Date: "2024-01-01", Weight: 155.0, BMI: 22.5},
+			},
+			settings: &models.Settings{
+				WeightUnit: "lbs",
+				HeightUnit: "in",
+				Height:     70,
+				GoalWeight: 150,
+			},
+			expectedStr: "Gained 10.00 lbs",
+		},
+		{
+			name: "no change shows 'No change'",
+			weights: []models.Weight{
+				{ID: 2, Date: "2024-01-08", Weight: 160.0, BMI: 23.0},
+				{ID: 1, Date: "2024-01-01", Weight: 160.0, BMI: 23.0},
+			},
+			settings: &models.Settings{
+				WeightUnit: "lbs",
+				HeightUnit: "in",
+				Height:     70,
+				GoalWeight: 150,
+			},
+			expectedStr: "No change",
+		},
+		{
+			name: "metric units show correctly",
+			weights: []models.Weight{
+				{ID: 2, Date: "2024-01-08", Weight: 70.0, BMI: 22.0},
+				{ID: 1, Date: "2024-01-01", Weight: 75.0, BMI: 23.0},
+			},
+			settings: &models.Settings{
+				WeightUnit: "kg",
+				HeightUnit: "cm",
+				Height:     180,
+				GoalWeight: 68,
+			},
+			expectedStr: "Lost 5.00 kg",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := display.RenderWeightsTable(tt.weights, tt.settings, 20)
+			if !strings.Contains(result, tt.expectedStr) {
+				t.Errorf("RenderWeightsTable() output does not contain expected delta string '%s'", tt.expectedStr)
+			}
+		})
+	}
+}