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.

tests/public-release-verifier.test.js

@@ -48,7 +48,7 @@ function createStage() {
   return stage;
 }
 
-function verify(stage, version = '2.0.6') {
+function verify(stage, version = '2.0.7') {
   return spawnSync(process.execPath, ['scripts/verify-public-release.mjs', '--source-only', '--version', version], {
     cwd: stage,
     encoding: 'utf8'