Add IntOrFalse type to handle polymorphic API responses

Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.

types_test.go

@@ -97,6 +97,31 @@ func TestStringInt64_UnmarshalJSON(t *testing.T) {
 	}
 }
 
+func TestIntOrFalse_UnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected int
+	}{
+		{"int value", `42`, 42},
+		{"int zero", `0`, 0},
+		{"false", `false`, 0},
+		{"true", `true`, 1},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var i IntOrFalse
+			if err := json.Unmarshal([]byte(tt.input), &i); err != nil {
+				t.Fatalf("unmarshal error: %v", err)
+			}
+			if int(i) != tt.expected {
+				t.Errorf("expected %v, got %v", tt.expected, i)
+			}
+		})
+	}
+}
+
 func TestProject_UnmarshalJSON(t *testing.T) {
 	jsonData := `{
 		"id": "1",