fix(parser): correct priority calculation for 'Z' to return 0

Previously, the priority calculation did not handle 'Z' correctly,
resulting in a non-zero value. Now, 'Z' returns 0 as intended, and the
step size is calculated for a linear scale.

internal/parser/parser.go

@@ -35,8 +35,12 @@ func (t Task) PriorityAsRemind() int {
 	if p < 'A' || p > 'Z' {
 		return 5000
 	}
-	// A=9999, Z=0, linear scale
-	return 9999 - int(p-'A')*400
+	step := 9999 / 25 // 399
+	val := 9999 - int(p-'A')*step
+	if p == 'Z' {
+		return 0
+	}
+	return val
 }
 
 func (t Task) MarshalJSON() ([]byte, error) {