chore: trivial fixes (#10650)

... which keep getting filed by AI agents.

Closes #10649, closes #10486, closes #10648, closes #10499, closes
#10647, closes #10635, closes #10636, closes #10607.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

cmd/syncthing/main.go

@@ -1054,7 +1054,7 @@ func (m migratingAPI) Serve(ctx context.Context) error {
 	srv := &http.Server{
 		Addr: m.addr,
 		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			w.Header().Set("Content-Type", "text/plain")
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 			w.Write([]byte("*** Database migration in progress ***\n\n"))
 			for _, line := range slogutil.GlobalRecorder.Since(time.Time{}) {
 				_, _ = line.WriteTo(w, slogutil.DefaultLineFormat)