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/router.go

@@ -0,0 +1,19 @@
+package main
+
+import "net/http"
+
+func newRouter() http.Handler {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", handleRoot)
+	mux.HandleFunc("/upload", handleUpload)
+	mux.HandleFunc("/select", handleSelect)
+	mux.HandleFunc("/process", handleProcess)
+	mux.HandleFunc("/progress", handleProgress)
+	mux.HandleFunc("/abort", handleAbort)
+	mux.HandleFunc("/pause", handlePause)
+	mux.HandleFunc("/resume", handleResume)
+	mux.HandleFunc("/clear-completed", handleClearCompleted)
+	mux.HandleFunc("/ws", handleWebSocket)
+	mux.HandleFunc("/set-speed-limit", handleSetSpeedLimit)
+	return withSecurityHeaders(mux)
+}