feat(app): support to edit screenshot area

- relocate or resize the area
- add anotations on the area

internal/application/annotation_test.go

@@ -0,0 +1,53 @@
+package application
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/draw"
+	"image/png"
+	"testing"
+)
+
+func TestApplyAnnotationsDrawsIntoPNG(t *testing.T) {
+	src := image.NewRGBA(image.Rect(0, 0, 40, 40))
+	draw.Draw(src, src.Bounds(), &image.Uniform{C: color.White}, image.Point{}, draw.Src)
+
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, src); err != nil {
+		t.Fatalf("encode source: %v", err)
+	}
+
+	out, err := ApplyAnnotations(buf.Bytes(), []Annotation{
+		{
+			Tool:  "rect",
+			Color: "#ef4444",
+			Points: []Point{
+				{X: 5, Y: 5},
+				{X: 30, Y: 30},
+			},
+		},
+	}, 1)
+	if err != nil {
+		t.Fatalf("apply annotations: %v", err)
+	}
+
+	img, err := png.Decode(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("decode output: %v", err)
+	}
+	if got := color.RGBAModel.Convert(img.At(5, 5)).(color.RGBA); got.R != 0xef || got.G != 0x44 || got.B != 0x44 {
+		t.Fatalf("expected annotated pixel at rectangle edge, got %#v", got)
+	}
+}
+
+func TestApplyAnnotationsNoAnnotationsReturnsOriginalBytes(t *testing.T) {
+	original := []byte("not decoded when no annotations")
+	out, err := ApplyAnnotations(original, nil, 1)
+	if err != nil {
+		t.Fatalf("apply annotations: %v", err)
+	}
+	if !bytes.Equal(out, original) {
+		t.Fatalf("expected original bytes")
+	}
+}