[V2] feat(split): add split mode options and custom page selection to section split (#5277)

# Description of Changes


This pull request adds a new "Split Mode" feature to the "Split by
Sections" tool, allowing users to customize how pages are split,
including options to exclude the first/last pages or specify custom
pages. The changes include updates to the UI, form data handling,
parameter definitions, and validation logic to support these new
options.

**Split by Sections: New Split Mode Feature**

*UI Enhancements:*
- Added a `Radio.Group` in `SplitSettings.tsx` to let users choose the
split mode (split all, exclude first/last, or custom pages), and a
conditional `TextInput` for custom page numbers when "Custom" is
selected.

*Localization:*
- Updated `translation.toml` with new labels and descriptions for the
split mode options and custom page numbers.

*Parameter Handling:*
- Extended the `SplitParameters` interface to include `splitMode` and
`customPages`, and set their default values.

*Form Submission:*
- Modified `buildSplitFormData` to include the selected split mode and
custom page numbers (if applicable) in the form data sent to the
backend.

*Validation Logic:*
- Updated the validation logic in `useSplitParameters` to require custom
page numbers when the custom split mode is selected.

<img width="363" height="998" alt="image"
src="https://github.com/user-attachments/assets/c9f781a6-cf7e-4f3d-8514-57cfe468e0c8"
/>




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Please provide a summary of the changes, including:

- What was changed
- 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)

### 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)

- [X] 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]>

frontend/public/locales/en-GB/translation.toml

@@ -3375,6 +3375,19 @@ placeholder = "Enter number of horizontal divisions"
 label = "Vertical Divisions"
 placeholder = "Enter number of vertical divisions"
 
+[split-by-sections.splitMode]
+label = "Split Mode"
+description = "Choose how to split the pages"
+splitAll = "Split all pages"
+splitAllExceptFirst = "Split all except first"
+splitAllExceptLast = "Split all except last"
+splitAllExceptFirstAndLast = "Split all except first and last"
+custom = "Custom pages"
+
+[split-by-sections.customPages]
+label = "Custom Page Numbers"
+placeholder = "e.g. 2,4,6"
+
 [AddStampRequest]
 tags = "Stamp, Add image, center image, Watermark, PDF, Embed, Customize,Customise"
 header = "Stamp PDF"