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/field.go

@@ -0,0 +1,36 @@
+package util
+
+type FieldUtil struct{}
+
+func (f FieldUtil) AllowSelect() []string {
+	return []string{
+		"download_folder",
+	}
+}
+
+func (f FieldUtil) AllowUpdate() []string {
+	return []string{
+		"download_folder",
+	}
+}
+
+func (f FieldUtil) IsAllow(allFields []string, names ...string) bool {
+	allowedFields := make(map[string]struct{})
+	for _, field := range allFields {
+		allowedFields[field] = struct{}{}
+	}
+	for _, name := range names {
+		if _, exists := allowedFields[name]; !exists {
+			return false
+		}
+	}
+	return true
+}
+
+func (f FieldUtil) IsAllowSelect(names ...string) bool {
+	return f.IsAllow(f.AllowSelect(), names...)
+}
+
+func (f FieldUtil) IsAllowUpdate(names ...string) bool {
+	return f.IsAllow(f.AllowUpdate(), names...)
+}