fix: resolve critical issues and improve task management

- Fix path traversal vulnerability in downloadVideo handler by adding
  download directory whitelist validation
- Add graceful shutdown with signal handling for task persistence
- Fix division by zero panic in progressBar.percent() when total <= 0
- Add GetDB() function that returns error instead of using log.Fatal
- Change deleteTask to only remove database records, preserve downloaded files
- Add paused status for interrupted tasks on shutdown

Co-Authored-By: Claude

internal/task/task.go

@@ -108,6 +108,18 @@ func CancelTask(taskID int64) bool {
 	return false
 }
 
+// MarkAllTasksPaused 标记所有活跃任务为暂停状态，用于程序退出时保存状态
+func MarkAllTasksPaused() {
+	GlobalTaskMux.Lock()
+	defer GlobalTaskMux.Unlock()
+	for _, task := range GlobalTaskList {
+		if task.Status == "waiting" || task.Status == "running" {
+			task.Cancelled = true
+			task.Status = "paused"
+		}
+	}
+}
+
 func (task *Task) Create(db *sql.DB) error {
 	util.SqliteLock.Lock()
 	result, err := db.Exec(`INSERT INTO "task" ("bvid", "cid", "format", "title", "owner", "cover", "status", "folder", "duration", "download_type")
@@ -463,6 +475,10 @@ func (p *progressBar) add(n int) {
 }
 
 func (p *progressBar) percent() float64 {
+	// 防止除零错误：total 为 0 或负数时返回 0
+	if p.total <= 0 {
+		return 0
+	}
 	return float64(p.current) / float64(p.total)
 }