Save signatures to server (#5080)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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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 <[email protected]>

app/core/src/test/java/stirling/software/SPDF/service/SignatureServiceTest.java

@@ -23,7 +23,7 @@ import stirling.software.common.configuration.InstallationPathConfig;
 class SignatureServiceTest {
 
     @TempDir Path tempDir;
-    private SignatureService signatureService;
+    private SharedSignatureService signatureService;
     private Path personalSignatureFolder;
     private Path sharedSignatureFolder;
     private final String ALL_USERS_FOLDER = "ALL_USERS";
@@ -53,7 +53,7 @@ class SignatureServiceTest {
                     .thenReturn(tempDir.toString());
 
             // Initialize the service with our temp directory
-            signatureService = new SignatureService();
+            signatureService = new SharedSignatureService();
         }
     }
 
@@ -165,7 +165,7 @@ class SignatureServiceTest {
     }
 
     @Test
-    void testGetSignatureBytes_PersonalFile() throws IOException {
+    void testGetSharedSignatureBytes_SharedFile() throws IOException {
         // Mock static method for each test
         try (MockedStatic<InstallationPathConfig> mockedConfig =
                 mockStatic(InstallationPathConfig.class)) {
@@ -173,28 +173,8 @@ class SignatureServiceTest {
                     .when(InstallationPathConfig::getSignaturesPath)
                     .thenReturn(tempDir.toString());
 
-            // Test
-            byte[] bytes = signatureService.getSignatureBytes(TEST_USER, "personal.png");
-
-            // Verify
-            assertEquals(
-                    "personal signature content",
-                    new String(bytes),
-                    "Should return the correct content for personal file");
-        }
-    }
-
-    @Test
-    void testGetSignatureBytes_SharedFile() throws IOException {
-        // Mock static method for each test
-        try (MockedStatic<InstallationPathConfig> mockedConfig =
-                mockStatic(InstallationPathConfig.class)) {
-            mockedConfig
-                    .when(InstallationPathConfig::getSignaturesPath)
-                    .thenReturn(tempDir.toString());
-
-            // Test
-            byte[] bytes = signatureService.getSignatureBytes(TEST_USER, "shared.jpg");
+            // Test - core service only reads shared signatures
+            byte[] bytes = signatureService.getSharedSignatureBytes("shared.jpg");
 
             // Verify
             assertEquals(
@@ -205,7 +185,7 @@ class SignatureServiceTest {
     }
 
     @Test
-    void testGetSignatureBytes_FileNotFound() {
+    void testGetSharedSignatureBytes_FileNotFound() {
         // Mock static method for each test
         try (MockedStatic<InstallationPathConfig> mockedConfig =
                 mockStatic(InstallationPathConfig.class)) {
@@ -216,13 +196,13 @@ class SignatureServiceTest {
             // Test and verify
             assertThrows(
                     FileNotFoundException.class,
-                    () -> signatureService.getSignatureBytes(TEST_USER, "nonexistent.png"),
+                    () -> signatureService.getSharedSignatureBytes("nonexistent.png"),
                     "Should throw exception for non-existent files");
         }
     }
 
     @Test
-    void testGetSignatureBytes_InvalidFileName() {
+    void testGetSharedSignatureBytes_InvalidFileName() {
         // Mock static method for each test
         try (MockedStatic<InstallationPathConfig> mockedConfig =
                 mockStatic(InstallationPathConfig.class)) {
@@ -233,11 +213,28 @@ class SignatureServiceTest {
             // Test and verify
             assertThrows(
                     IllegalArgumentException.class,
-                    () -> signatureService.getSignatureBytes(TEST_USER, "../invalid.png"),
+                    () -> signatureService.getSharedSignatureBytes("../invalid.png"),
                     "Should throw exception for file names with directory traversal");
         }
     }
 
+    @Test
+    void testGetSharedSignatureBytes_CannotAccessPersonalFiles() {
+        // Mock static method for each test
+        try (MockedStatic<InstallationPathConfig> mockedConfig =
+                mockStatic(InstallationPathConfig.class)) {
+            mockedConfig
+                    .when(InstallationPathConfig::getSignaturesPath)
+                    .thenReturn(tempDir.toString());
+
+            // Test and verify - core service should NOT be able to read personal files
+            assertThrows(
+                    FileNotFoundException.class,
+                    () -> signatureService.getSharedSignatureBytes("personal.png"),
+                    "Core service should not have access to personal signatures");
+        }
+    }
+
     @Test
     void testGetAvailableSignatures_EmptyUsername() throws IOException {
         // Mock static method for each test