appview: repo: trim leading @ from collaborator's handle

The "add collaborator" form currently fails if you submit a handle like `@example.com`, since the identity gets forwarded eventually directly to Bluesky's identity parse, which does not accept the `@` prefix for handles.

This change is targetted to only fix that one form.

Signed-off-by: phil <[e-mail]>

appview/repo/repo.go

···
 		return
 	}
 
+	// remove a single leading `@`, to make @handle work with ResolveIdent
+	collaborator = strings.TrimPrefix(collaborator, "@")
+
 	collaboratorIdent, err := rp.idResolver.ResolveIdent(r.Context(), collaborator)
 	if err != nil {
 		fail(fmt.Sprintf("'%s' is not a valid DID/handle.", collaborator), err)