feat(common): add configurable maxDPI limit for PDF-to-image conversion (#4129)

# Description of Changes

- **What was changed:**  
Added a new `maxDPI` property under `system` in `ApplicationProperties`;
updated `PdfUtils` to retrieve and enforce this configurable limit
instead of a hard-coded constant; modified `ConverterWebController` and
the PDF-to-image template to expose the limit to users; added
`pdfToImage.dpi` entries across all translation files; updated
`settings.yml.template` and `DeveloperGuide.md` to document the new
setting.
- **Why the change was made:**  
To allow deployments to tune the maximum DPI for PDF-to-image
conversions based on available resources, preventing excessive memory
usage and crashes caused by arbitrarily high DPI values.

Closes #3985

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/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)
- [x] I have performed a self-review of my own code
- [x] 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)

### 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 tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/core/src/main/java/stirling/software/SPDF/controller/web/ConverterWebController.java

@@ -8,6 +8,8 @@ import org.springframework.web.servlet.ModelAndView;
 import io.swagger.v3.oas.annotations.Hidden;
 import io.swagger.v3.oas.annotations.tags.Tag;
 
+import stirling.software.common.model.ApplicationProperties;
+import stirling.software.common.util.ApplicationContextProvider;
 import stirling.software.common.util.CheckProgramInstall;
 
 @Controller
@@ -62,6 +64,13 @@ public class ConverterWebController {
     @Hidden
     public String pdfToimgForm(Model model) {
         boolean isPython = CheckProgramInstall.isPythonAvailable();
+        ApplicationProperties properties =
+                ApplicationContextProvider.getBean(ApplicationProperties.class);
+        if (properties != null && properties.getSystem() != null) {
+            model.addAttribute("maxDPI", properties.getSystem().getMaxDPI());
+        } else {
+            model.addAttribute("maxDPI", 500); // Default value if not set
+        }
         model.addAttribute("isPython", isPython);
         model.addAttribute("currentPage", "pdf-to-img");
         return "convert/pdf-to-img";