chore: reformat Java codebase and centralize Spotless config (#3723)

# Description of Changes

Please provide a summary of the changes, including:

- Extracted the `googleJavaFormatVersion` into a centralized Gradle
property for easier management across modules.
- Added consistent `spotless` formatting configuration to `common`,
`proprietary`, and `stirling-pdf` modules.
- Applied automatic import ordering and removed unused imports in
numerous Java files.
- Reordered and grouped imports consistently, improving overall code
readability.
- Removed excessive blank lines and standardized spacing.
- Ensured a uniform coding style throughout the codebase using Spotless
and Google Java Format with AOSP style.

---

## 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)
- [ ] 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
- [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/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.

common/build.gradle

@@ -2,7 +2,12 @@
 bootRun {
     enabled = false
 }
-
+spotless {
+    java {
+        target sourceSets.main.allJava
+        googleJavaFormat(googleJavaFormatVersion).aosp()
+    }
+}
 dependencies {
     api 'org.springframework.boot:spring-boot-starter-web'
     api 'org.springframework.boot:spring-boot-starter-thymeleaf'
@@ -17,4 +22,4 @@ dependencies {
     api 'org.snakeyaml:snakeyaml-engine:2.9'
     api "org.springdoc:springdoc-openapi-starter-webmvc-ui:2.8.8"
     api 'jakarta.mail:jakarta.mail-api:2.1.3'
-}
+}