i18n(close_zen_button): add a translatable title to the button

previously it only displayed the keyboard shortcut, without any extra info

src/public/app/widgets/close_zen_button.ts

@@ -1,9 +1,12 @@
 import BasicWidget from "./basic_widget.js";
+import { t } from "../services/i18n.js";
 
 const TPL = `\
 <div class="close-zen-container">
     <button class="button-widget bx icon-action bxs-yin-yang"
-        data-trigger-command="toggleZenMode" />
+        data-trigger-command="toggleZenMode"
+        title="${t("zen_mode.button_exit")}"
+    />
 
     <style>
     :root {

src/public/translations/en/translation.json

@@ -645,6 +645,9 @@
     "show-cheatsheet": "Show Cheatsheet",
     "toggle-zen-mode": "Zen Mode"
   },
+  "zen_mode": {
+    "button_exit": "Exit Zen Mode"
+  },
   "sync_status": {
     "unknown": "<p>Sync status will be known once the next sync attempt starts.</p><p>Click to trigger sync now.</p>",
     "connected_with_changes": "<p>Connected to the sync server. <br>There are some outstanding changes yet to be synced.</p><p>Click to trigger sync.</p>",