fix(notes): export the notes in a stable order

AllNotes returns newest-first by the second, so two notes written in the same
second have no order between them — and the notes export walks that list assigning
filenames, giving the first note of a name the plain filename and the next the
"(2)". With the order undecided, which of two same-titled notes got which name was
a coin toss that landed differently on each export, and the export test failed
roughly one run in three.

Sort the export by when each note was made, with the id to break a same-second
tie. The first note of a name keeps the name; the archive is the same twice
running. Ids are time-ordered, so the tie-break agrees with creation order rather
than fighting it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/notes_handlers.go

@@ -313,6 +313,18 @@ func (s *Server) handleExportNotes(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Oldest first, and the id to break a tie. AllNotes comes back newest-first by
+	// the second, so two notes made in the same second have no order between them —
+	// and then which of two same-titled notes gets the plain name and which gets
+	// "(2)" is a coin toss that lands differently each export. Ordering the whole
+	// thing by when it was made settles it: the first note of a name keeps the name.
+	sort.SliceStable(notes, func(i, j int) bool {
+		if notes[i].CreatedAt.Equal(notes[j].CreatedAt) {
+			return notes[i].ID < notes[j].ID
+		}
+		return notes[i].CreatedAt.Before(notes[j].CreatedAt)
+	})
+
 	w.Header().Set("Content-Type", "application/zip")
 	w.Header().Set("Content-Disposition",
 		`attachment; filename="verdande-noter-`+time.Now().Format("2006-01-02")+`.zip"`)