fix update notification visibility and install flicker (#6776)

# Description of Changes

- Closes #6754
- Update popup now hidden on mobile, for non-admins, and never on SaaS
- Respects admin "Show Update Notifications" setting (`showUpdate` /
`showUpdateOnlyAdmin`, now default on)
- Fixes update modal flickering during desktop install
---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -869,8 +869,8 @@ public class ApplicationProperties {
     public static class System {
         private String defaultLocale;
         private boolean googlevisibility;
-        private boolean showUpdate;
-        private boolean showUpdateOnlyAdmin;
+        private boolean showUpdate = true;
+        private boolean showUpdateOnlyAdmin = true;
         private boolean showSettingsWhenNoLogin = true;
         private boolean customHTMLFiles;
         private String tessdataDir;