[V2] feat(convert): add PDF/X export option (#5285)

# Description of Changes

This pull request adds support for converting PDF files to PDF/X format
in the frontend, alongside the existing PDF/A conversion. The changes
include UI updates, translation strings, configuration, and processing
logic to enable PDF to PDF/X conversion using the same backend endpoint
as PDF/A, with appropriate user options and warnings.

**PDF/X Conversion Support:**

- Added a new `ConvertToPdfxSettings` component for configuring PDF/X
conversion options, including output format selection and digital
signature warnings.
(`frontend/src/core/components/tools/convert/ConvertToPdfxSettings.tsx`)
- Updated the conversion settings UI to display PDF/X options when
converting from PDF to PDF/X.
(`frontend/src/core/components/tools/convert/ConvertSettings.tsx`)

**Configuration and Processing Logic:**

- Extended conversion constants and parameters to recognize PDF/X as a
target format, map it to the PDF/A backend endpoint, and ensure correct
file naming and processing behavior.
(`frontend/src/core/constants/convertConstants.ts`,
`frontend/src/core/hooks/tools/convert/useConvertOperation.ts`,
`frontend/src/core/hooks/tools/convert/useConvertParameters.ts`)
- Ensured that PDF/X conversions are processed separately per file,
similar to PDF/A.
(`frontend/src/core/hooks/tools/convert/useConvertOperation.ts`)

**UI and Translation Updates:**

- Updated translation strings to include PDF/X options, descriptions,
and warnings. (`frontend/public/locales/en-GB/translation.toml`)

<img width="366" height="998" alt="image"
src="https://github.com/user-attachments/assets/b28fa095-9350-4db2-a0b5-bddcf003fa46"
/>


<!--
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.

---------

Signed-off-by: Balázs Szücs <[email protected]>
Co-authored-by: Anthony Stirling <[email protected]>

frontend/src/core/tests/convert/ConvertIntegration.test.tsx

@@ -152,6 +152,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -231,6 +234,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -288,6 +294,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -354,6 +363,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -424,6 +436,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -492,6 +507,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -556,6 +574,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -617,6 +638,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -680,6 +704,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -740,6 +767,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -806,6 +836,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },
@@ -871,6 +904,9 @@ describe('Convert Tool Integration Tests', () => {
         pdfaOptions: {
           outputFormat: ''
         },
+        pdfxOptions: {
+          outputFormat: 'pdfx'
+        },
         cbrOptions: {
           optimizeForEbook: false
         },