refactor: restructure project to standard Go layout

- Reorganize code into cmd/bilidown and internal/ packages
- Rename client/ to web/ for frontend source
- Remove systray dependency for headless web service
- Embed static files into binary using go:embed
- Update import paths to use internal/ prefix
- Update .gitignore with common patterns

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/util/util_test.go

@@ -0,0 +1,29 @@
+package util_test
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"bilidown/common"
+	"bilidown/util"
+)
+
+func TestRandomString(t *testing.T) {
+	for i := 4; i < 10; i++ {
+		for j := 0; j < 3; j++ {
+			str := common.RandomString(i)
+			t.Log(str)
+		}
+		t.Log("\n")
+	}
+}
+
+func TestGetRedirectedLocation(t *testing.T) {
+	os.Setenv("https_proxy", "http://192.168.1.5:9000")
+	if location, err := util.GetRedirectedLocation("https://b23.tv/Ga6sbzT"); err != nil {
+		t.Error(err)
+	} else {
+		fmt.Println(location)
+	}
+}