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/SPDFApplication.java

@@ -83,18 +83,17 @@ public class SPDFApplication {
         Map<String, String> propertyFiles = new HashMap<>();
 
         // External config files
-        log.info("Settings file: {}", InstallationPathConfig.getSettingsPath());
-        if (Files.exists(Paths.get(InstallationPathConfig.getSettingsPath()))) {
-            propertyFiles.put(
-                    "spring.config.additional-location",
-                    "file:" + InstallationPathConfig.getSettingsPath());
+        String settingsPath = InstallationPathConfig.getSettingsPath();
+        log.info("Settings file: {}", settingsPath);
+        if (Files.exists(Paths.get(settingsPath))) {
+            propertyFiles.put("spring.config.additional-location", "file:" + settingsPath);
         } else {
-            log.warn(
-                    "External configuration file '{}' does not exist.",
-                    InstallationPathConfig.getSettingsPath());
+            log.warn("External configuration file '{}' does not exist.", settingsPath);
         }
 
-        if (Files.exists(Paths.get(InstallationPathConfig.getCustomSettingsPath()))) {
+        String customSettingsPath = InstallationPathConfig.getCustomSettingsPath();
+        log.info("Custom settings file: {}", customSettingsPath);
+        if (Files.exists(Paths.get(customSettingsPath))) {
             String existingLocation =
                     propertyFiles.getOrDefault("spring.config.additional-location", "");
             if (!existingLocation.isEmpty()) {
@@ -102,11 +101,9 @@ public class SPDFApplication {
             }
             propertyFiles.put(
                     "spring.config.additional-location",
-                    existingLocation + "file:" + InstallationPathConfig.getCustomSettingsPath());
+                    existingLocation + "file:" + customSettingsPath);
         } else {
-            log.warn(
-                    "Custom configuration file '{}' does not exist.",
-                    InstallationPathConfig.getCustomSettingsPath());
+            log.warn("Custom configuration file '{}' does not exist.", customSettingsPath);
         }
         Properties finalProps = new Properties();
 
@@ -128,7 +125,7 @@ public class SPDFApplication {
         try {
             Files.createDirectories(Path.of(InstallationPathConfig.getTemplatesPath()));
             Files.createDirectories(Path.of(InstallationPathConfig.getStaticPath()));
-        } catch (Exception e) {
+        } catch (IOException e) {
             log.error("Error creating directories: {}", e.getMessage());
         }