Save signatures to server (#5080)

# Description of Changes

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- Why the change was made
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] 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)
- [ ] 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/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>

app/common/src/main/java/stirling/software/common/service/PersonalSignatureServiceInterface.java

@@ -0,0 +1,21 @@
+package stirling.software.common.service;
+
+import java.io.IOException;
+
+/**
+ * Interface for personal signature access (proprietary feature). Implemented only in proprietary
+ * module to provide authenticated users access to their personal signatures.
+ */
+public interface PersonalSignatureServiceInterface {
+
+    /**
+     * Get a personal signature from the user's folder. Only checks personal folder, not shared
+     * folder.
+     *
+     * @param username Username of the signature owner
+     * @param fileName Signature filename
+     * @return Personal signature image bytes
+     * @throws IOException If file not found or read error
+     */
+    byte[] getPersonalSignatureBytes(String username, String fileName) throws IOException;
+}