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/res_error/res_error.go

@@ -0,0 +1,26 @@
+package res_error
+
+import (
+	"net/http"
+
+	"bilidown/internal/util"
+)
+
+// Send 发送异常响应
+func Send(w http.ResponseWriter, message string) {
+	util.Res{Message: message, Success: false}.Write(w)
+}
+
+const (
+	BvidFormatError       = "错误的 Bvid 格式"
+	URLFormatError        = "错误的 URL 格式"
+	MidFormatError        = "错误的 Mid 格式"
+	SeasonIdFormatError   = "错误的 SeasonId 格式"
+	ParamError            = "参数错误"
+	MethodNotAllowError   = "不允许的请求方式"
+	NoLocationError       = "无重定向目标地址"
+	FileNotFountError     = "文件不存在"
+	FileTypeNotAllowError = "不允许的文件类型"
+	SystemError           = "系统错误"
+	NotLogin              = "未登录"
+)