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.

build.gradle

@@ -27,6 +27,7 @@ ext {
     springSecuritySamlVersion = "6.5.0"
     openSamlVersion = "4.3.2"
     commonmarkVersion = "0.24.0"
+    googleJavaFormatVersion = "1.27.0"
     tempJrePath = null
 }
 
@@ -475,7 +476,7 @@ spotless {
         target project(':proprietary').sourceSets.main.allJava
         target project(':stirling-pdf').sourceSets.main.allJava
 
-        googleJavaFormat("1.27.0").aosp().reorderImports(false)
+        googleJavaFormat(googleJavaFormatVersion).aosp().reorderImports(false)
 
         importOrder("java", "javax", "org", "com", "net", "io", "jakarta", "lombok", "me", "stirling")
         toggleOffOn()