scp80: keep the card preset selected after a send (v2.1.18)

cardsSave() calls cardsRebuildSelect(), and the innerHTML rebuild reset the
dropdown to its first option ("- Select card -"). Since the counter sync
saves after every send, the selection was lost each time and had to be
re-picked manually. The rebuild now restores the previous selection
(guarded by the preset still existing). Service worker v155.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v154';
+const CACHE = 'otaman-v155';
 const URLS = [
   'index.html',
   'help.html',