Feature/v2/landing page (#4128)

# Description of Changes

- Added landing page styling
- Clicking add files now opens native file manager

---

## 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)
- [ ] 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)

### 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.

frontend/public/branding/StirlingPDFLogoNoTextDark.svg

@@ -0,0 +1,4 @@
+<svg width="146" height="157" viewBox="0 0 146 157" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M3.77397 72.5462L93.6741 23.0462L94.7739 70.0462L3.77395 119.046L3.77397 72.5462Z" fill="#E6E6E6" fill-opacity="0.9"/>
+<path d="M50.774 73.5735L96.387 50.2673L142 26.961L142 71.687L50.7739 122.046L50.774 73.5735Z" fill="#E6E6E6" fill-opacity="0.8"/>
+</svg>

frontend/public/branding/StirlingPDFLogoNoTextLight.svg

@@ -0,0 +1,4 @@
+<svg width="146" height="146" viewBox="0 0 146 146" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M3.77397 72.5462L93.6741 23.0462L94.7739 70.0462L3.77395 119.046L3.77397 72.5462Z" fill="#ACACAC" fill-opacity="0.3"/>
+<path d="M50.774 73.5735L96.387 50.2673L142 26.961L142 71.687L50.7739 122.046L50.774 73.5735Z" fill="#FC9999" fill-opacity="0.5"/>
+</svg>