feat: add FFmpeg status check and shutdown button

- FFmpeg check no longer blocks server startup
- Add /api/checkFFmpeg endpoint to query FFmpeg availability
- Add /api/shutdown endpoint to gracefully stop the server
- Add FFmpeg status indicator in web header
- Add shutdown button in web interface
- Move static files output to cmd/bilidown/static for embedding
- Update .gitignore to exclude cmd/bilidown/static

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/util/db.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"log"
-	"os"
 	"strings"
 	"sync"
 )
@@ -97,37 +96,19 @@ func GetCurrentFolder(db *sql.DB) (string, error) {
 		if err != nil {
 			return "", err
 		}
-		err = os.MkdirAll(folder, os.ModePerm)
-		if err != nil {
-			return "", err
-		}
 		err = SaveDownloadFolder(db, folder)
 		if err != nil {
 			return "", err
 		}
 		return folder, nil
 	}
-	err = os.MkdirAll(folder, os.ModePerm)
-	if err != nil {
-		return "", err
-	}
-	return folder, nil
+	return folder, err
 }
 
-// SaveDownloadFolder 保存下载路径，不存在则自动创建
+// SaveDownloadFolder 保存下载路径（不自动创建目录）
 func SaveDownloadFolder(db *sql.DB, downloadFolder string) error {
-	_, err := os.Stat(downloadFolder)
-	if err != nil {
-		if os.IsNotExist(err) {
-			err = os.MkdirAll(downloadFolder, os.ModePerm)
-			if err != nil {
-				return err
-			}
-		}
-		return err
-	}
 	SqliteLock.Lock()
-	_, err = db.Exec(`INSERT OR REPLACE INTO "field" ("name", "value") VALUES ('download_folder', ?)`, downloadFolder)
+	_, err := db.Exec(`INSERT OR REPLACE INTO "field" ("name", "value") VALUES ('download_folder', ?)`, downloadFolder)
 	SqliteLock.Unlock()
 	return err
 }