Harden web/download pipeline and split handler modules

Replace shell-based downloader execution with validated arguments, enforce request hardening and safer defaults, and refactor handlers/router/state so job control is safer and easier to maintain.

src/handlers_common.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"net/url"
+	"regexp"
+	"strings"
+)
+
+type ProgressInfo struct {
+	Percentage  float64
+	CurrentFile string
+	Paused      bool
+	Status      string
+}
+
+var speedLimitRegex = regexp.MustCompile(`^([1-9]\d*(\.\d+)?)(KBps|MBps|GBps)$`)
+
+func withToken(path string) string {
+	token := strings.TrimSpace(config.Security.AuthToken)
+	if token == "" {
+		return path
+	}
+
+	separator := "?"
+	if strings.Contains(path, "?") {
+		separator = "&"
+	}
+
+	return path + separator + "token=" + url.QueryEscape(token)
+}