feat(merge): use metadata for sortByDate (#4461)

# Description of Changes

Follow-up for:
https://github.com/Stirling-Tools/Stirling-PDF/issues/4233#issuecomment-3299735631

Please read thread there.

### Quick summary:
- Paths/BasicFileAttributes for getting creaton/modifcation date ALWAYS
resulted on 0
- Comporator, therefore always merged in the order it was handed to it
- -\> when calling this was not an issue because front-end arranged the
files on right sortByDate
- -\> when calling from API however, due to this, it wasn't working

### Fix:
- Rely on PDF/XMP metadata on the back-end for merging, while that is
also not completely reliable still better

Closes: #4233
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- Why the change was made
- Any challenges encountered

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---

## 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/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)
- [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/devGuide/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)

- [x] 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.

---------

Signed-off-by: Balázs Szücs <[email protected]>

app/core/src/main/resources/static/js/merge.js

@@ -175,6 +175,13 @@ function updateFiles() {
     }
   }
   document.getElementById("fileInput-input").files = dataTransfer.files;
+
+  // Also populate hidden fileOrder to preserve visible order
+  const order = Array.from(liElements)
+    .map((li) => li.querySelector(".filename").innerText)
+    .join("\n");
+  const orderInput = document.getElementById("fileOrder");
+  if (orderInput) orderInput.value = order;
 }
 
 document.querySelector("#resetFileInputBtn").addEventListener("click", ()=>{

app/core/src/main/resources/templates/merge-pdfs.html

@@ -21,6 +21,7 @@
               <span class="tool-header-text" th:text="#{merge.header}"></span>
             </div>
             <form th:action="@{'api/v1/general/merge-pdfs'}" method="post" enctype="multipart/form-data">
+              <input id="fileOrder" name="fileOrder" type="hidden" />
               <div class="mb-3">
                 <label th:text="#{multiPdfDropPrompt}" for="fileInput-input"></label>
                 <div