fix: comment url swaps title and answer id

[MaxFreedomPollard]

Proposed Changes

A comment left on an answer links to the wrong URL. CommentURL calls its sibling AnswerURL with the last two arguments reversed:

// pkg/display/url.go:57
return AnswerURL(permalink, siteUrl, questionID, answerID, title) + "?commentId=" + commentID

// pkg/display/url.go:44
func AnswerURL(permalink int, siteUrl, questionID, title, answerID string) string

The answer route is /questions/:id/:title/:answerid (internal/router/template_router.go:71, and questions/:qid/:slugPermalink/:aid in ui/src/router/routes.ts:116), so the answer id ends up in the title slot and the title ends up in the answer id slot. What the title then becomes depends on the permalink setting: the numeric ones send it through uid.DeShortID, which parses it as a short id and returns an unrelated number, and the short id ones send it through uid.EnShortID, which returns non-numeric input unchanged, so the raw title with its spaces is written straight into the path.

For question 10010000000000001, title How to install Answer, answer 10020000000000002, comment 10030000000000003:

permalink 1  got  /questions/10010000000000001/10020000000000002/1846951505924529392?commentId=10030000000000003
             want /questions/10010000000000001/how-to-install-answer/10020000000000002?commentId=10030000000000003
permalink 2  got  /questions/10010000000000001/1846951505924529392?commentId=10030000000000003
             want /questions/10010000000000001/10020000000000002?commentId=10030000000000003
permalink 3  got  /questions/D1D1/10020000000000002/How to install Answer?commentId=10030000000000003
             want /questions/D1D1/how-to-install-answer/E1E1?commentId=10030000000000003
permalink 4  got  /questions/D1D1/How to install Answer?commentId=10030000000000003
             want /questions/D1D1/E1E1?commentId=10030000000000003

This is what readers click in the new comment notification email built in internal/service/export/email_service.go:314, and it is the CommentUrl handed to notification plugins in internal/service/notification_common/notification.go:399. Both call sites already pass the arguments in the order CommentURL declares, so the swap is entirely inside CommentURL. A comment on a question takes the other branch, which was already correct.

• Pass title and answerID to AnswerURL in the order it declares them, so a comment on an answer is the answer URL plus the comment query
• Add pkg/display/url_test.go covering both branches across all four permalink settings

Testing

Against dev at 947a48b, the new test fails on all four permalink settings for a comment on an answer, and the four question cases pass, which is the branch that was already correct. With the one line changed:

go test ./pkg/display/ -v   ok, 8 subtests pass
go test ./pkg/...           all packages ok
go build ./...              ok
go vet ./pkg/display/... ./internal/service/export/... ./internal/service/notification_common/...   ok
golangci-lint run ./pkg/display/...  (v2.6.2, repo .golangci.yaml)   0 issues
go mod tidy                 no change to go.mod or go.sum

No existing test referenced CommentURL, so nothing else needed updating. The test uses an ASCII title, which htmltext.UrlTitle slugifies identically regardless of the transliterateNonLatin setting, locale or timezone.

[LinkinStars]

Thank you very much for your contribution. This is indeed a bug.