Refactor Path Handling (#3041)

# Description of Changes

Please provide a summary of the changes, including:

What was changed:

- Refactored path constructions in multiple classes (e.g.,
SPDFApplication.java, InstallationPathConfig.java,
RuntimePathConfig.java) to use Java NIO’s Paths.get() and Path.of()
instead of manual string concatenation.


Why the change was made:

- To improve code readability, maintainability, and robustness by
leveraging modern Java NIO utilities.
- To ensure better portability across different operating systems by
avoiding hardcoded file separators.


Challenges encountered:

- Maintaining backward compatibility while transitioning from manual
string concatenation to using Paths for file path construction.
- Ensuring that the refactored path resolution works consistently across
all supported environments (Windows, macOS, Linux, and Docker).

@Frooodle can you check the docker path `/.dockerenv`?

---

## 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/DeveloperGuide.md)
(if applicable)
- [x] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] 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/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/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/config/security/database/DatabaseConfig.java

@@ -1,7 +1,5 @@
 package stirling.software.SPDF.config.security.database;
 
-import java.io.File;
-
 import javax.sql.DataSource;
 
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -37,8 +35,8 @@ public class DatabaseConfig {
         DATASOURCE_DEFAULT_URL =
                 "jdbc:h2:file:"
                         + InstallationPathConfig.getConfigPath()
-                        + File.separator
                         + "stirling-pdf-DB-2.3.232;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=FALSE";
+        log.debug("Database URL: {}", DATASOURCE_DEFAULT_URL);
         this.applicationProperties = applicationProperties;
         this.runningEE = runningEE;
     }