Add FFmpeg utilities configuration UI and automated downloading

Introduce a new UI for configuring FFmpeg, FFprobe, and FFplay paths with file selection and error handling. Add platform-specific logic for downloading and extracting FFmpeg binaries directly within the application, improving user experience.

internal/utils/path_separator.go

@@ -0,0 +1,8 @@
+//go:build !windows
+// +build !windows
+
+package utils
+
+func PathSeparator() string {
+	return "/"
+}