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/ffmpeg/download/service/download.go

@@ -0,0 +1,21 @@
+package service
+
+import (
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/canvas"
+	"fyne.io/fyne/v2/widget"
+	"git.kor-elf.net/kor-elf/gui-for-ffmpeg/internal/application"
+	"git.kor-elf.net/kor-elf/gui-for-ffmpeg/internal/ffmpeg/download/gui"
+)
+
+func DownloadFFmpeg(app application.AppContract, save func(ffmpegPath string, ffprobePath string, ffplayPath string) error) fyne.CanvasObject {
+	return gui.DownloadFFmpeg(func(progressBar *widget.ProgressBar, progressMessage *canvas.Text) error {
+		var err error
+		err = startDownload(app, progressBar, progressMessage, save)
+		if err != nil {
+			return err
+		}
+
+		return nil
+	})
+}