feat(app): implement basic function

General
- 配置界面支持配置对象存储桶
- 快捷键进入截图
- 一键上传截图 & 复制截图链接

MacOS
- 状态栏指示器和应用图标

internal/infrastructure/screencapture/cropper.go

@@ -0,0 +1,59 @@
+package screencapture
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/png"
+)
+
+// CropPNG decodes the supplied PNG, crops it to rect (in PHYSICAL pixels of
+// the source image — i.e. the same coordinate system the PNG itself uses),
+// and re-encodes the result.
+//
+// Why an extra abstraction: the overlay flow captures the entire screen as
+// one big PNG and lets the user pick a region in the WebView. The selection
+// rectangle the WebView reports is in CSS pixels, so callers must scale by
+// the display's backing factor before invoking CropPNG. Splitting "scale"
+// from "crop" keeps this helper focused and unit-testable.
+func CropPNG(pngBytes []byte, rect image.Rectangle) ([]byte, error) {
+	if rect.Dx() <= 0 || rect.Dy() <= 0 {
+		return nil, fmt.Errorf("invalid crop rectangle: %v", rect)
+	}
+	img, err := png.Decode(bytes.NewReader(pngBytes))
+	if err != nil {
+		return nil, fmt.Errorf("decode source png: %w", err)
+	}
+	// Clip the requested rectangle to the source bounds — defensive guard
+	// against off-by-one errors from CSS-px → physical-px scaling.
+	src := img.Bounds()
+	clipped := rect.Intersect(src)
+	if clipped.Empty() {
+		return nil, fmt.Errorf("crop rectangle %v has no overlap with source %v", rect, src)
+	}
+
+	// SubImage is documented on most concrete image types; the standard
+	// library decodes PNGs into one of those, so this assertion is safe in
+	// practice. Fall back to a manual copy otherwise.
+	type subImager interface {
+		SubImage(r image.Rectangle) image.Image
+	}
+	var cropped image.Image
+	if si, ok := img.(subImager); ok {
+		cropped = si.SubImage(clipped)
+	} else {
+		dst := image.NewRGBA(image.Rect(0, 0, clipped.Dx(), clipped.Dy()))
+		for y := 0; y < clipped.Dy(); y++ {
+			for x := 0; x < clipped.Dx(); x++ {
+				dst.Set(x, y, img.At(clipped.Min.X+x, clipped.Min.Y+y))
+			}
+		}
+		cropped = dst
+	}
+
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, cropped); err != nil {
+		return nil, fmt.Errorf("encode cropped png: %w", err)
+	}
+	return buf.Bytes(), nil
+}