Use a dropdown and a Create ticket button on the ticket panel, in orange

The panel is now one category dropdown and a Create ticket button
instead of a button per category. Discord doesn't send the dropdown's
value with a button click, so each member's pick is remembered in
memory for 15 minutes (per guild and member, capped). Panels posted
before keep working until they're reposted. Every ticket embed is
orange, closed tickets included.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/tickets/manage.go

@@ -182,7 +182,7 @@ func postLog(ctx context.Context, d Discord, st Store, t *ticketrepo.Ticket, tra
 	embed := &discordgo.MessageEmbed{
 		Title:       fmt.Sprintf("Ticket #%04d closed · %s", t.Number, clip(t.CategoryName, 60)),
 		Description: "**" + clip(t.Subject, maxSubject) + "**",
-		Color:       0x6b7280,
+		Color:       accent,
 		Fields: []*discordgo.MessageEmbedField{
 			{Name: "Opened by", Value: "<@" + id(t.OpenerID) + ">", Inline: true},
 			{Name: "Claimed by", Value: claimed, Inline: true},