fix(updater): show header action only when update is available

Hide the header update action until a newer release is confirmed.
Register update notifications before renderer initialization so startup cannot miss them.
Refresh the public product screenshot and release metadata for v2.0.7.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "multi-hoster-uploader",
-  "version": "2.0.6",
+  "version": "2.0.7",
   "description": "Upload files to doodstream, voe, vidmoly, byse simultaneously",
   "main": "main.js",
   "scripts": {