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/src/styles/theme.css

@@ -106,6 +106,30 @@
   /* Inactive icon colors for light mode */
   --icon-inactive-bg: #9CA3AF;
   --icon-inactive-color: #FFFFFF;
+
+  --accent-interactive: #4A90E2;
+  --text-instruction: #4A90E2;
+  --text-brand: var(--color-gray-700);
+  --text-brand-accent: #DC2626;
+
+  /* container */
+  --landing-paper-bg: var(--bg-surface);
+  --landing-inner-paper-bg: #EEF8FF;
+  --landing-inner-paper-border: #CDEAFF;
+  --landing-button-bg: var(--bg-surface);
+  --landing-button-color: var(--icon-tools-bg);
+  --landing-button-border: #E0F2F7;
+  --landing-button-hover-bg: rgb(251, 251, 251);
+
+  /* drop state */
+  --landing-drop-paper-bg: #E3F2FD;
+  --landing-drop-inner-paper-bg: #BBDEFB;
+  --landing-drop-inner-paper-border: #90CAF9;
+
+  /* shadows */
+  --drop-shadow-color: rgba(0, 0, 0, 0.08);
+  --drop-shadow-color-strong: rgba(0, 0, 0, 0.04);
+  --drop-shadow-filter: drop-shadow(0 0.2rem 0.4rem rgba(0, 0, 0, 0.08)) drop-shadow(0 0.6rem 0.6rem rgba(0, 0, 0, 0.06)) drop-shadow(0 1.2rem 1rem rgba(0, 0, 0, 0.04));
 }
 
 [data-mantine-color-scheme="dark"] {
@@ -177,6 +201,29 @@
   --icon-inactive-bg: #2A2F36;
   --icon-inactive-color: #6E7581;
 
+  --accent-interactive: #ffffff;
+  --text-instruction: #ffffff;
+  --text-brand: var(--color-gray-800);
+  --text-brand-accent: #EF4444;
+
+  /* container */
+  --landing-paper-bg: #171A1F;
+  --landing-inner-paper-bg: var(--bg-raised);
+  --landing-inner-paper-border: #2D3237;
+  --landing-button-bg: #2B3037;
+  --landing-button-color: #ffffff;
+  --landing-button-border: #2D3237;
+  --landing-button-hover-bg: #4c525b;
+
+  /* drop state */
+  --landing-drop-paper-bg: #1A2332;
+  --landing-drop-inner-paper-bg: #2A3441;
+  --landing-drop-inner-paper-border: #3A4451;
+
+  /* shadows */
+  --drop-shadow-color: rgba(255, 255, 255, 0.08);
+  --drop-shadow-color-strong: rgba(255, 255, 255, 0.04);
+  --drop-shadow-filter: drop-shadow(0 0.2rem 0.4rem rgba(200, 200, 200, 0.08)) drop-shadow(0 0.6rem 0.6rem rgba(200, 200, 200, 0.06)) drop-shadow(0 1.2rem 1rem rgba(200, 200, 200, 0.04));
   /* Adjust shadows for dark mode */
   --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.3);
   --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.4);
@@ -185,6 +232,12 @@
   --shadow-xl: 0 20px 25px rgba(0, 0, 0, 0.4);
 }
 
+/* Dropzone drop state styling */
+[data-accept] .dropzone-inner {
+  background-color: var(--landing-drop-inner-paper-bg) !important;
+  border-color: var(--landing-drop-inner-paper-border) !important;
+}
+
 /* Smooth transitions for theme switching */
 * {
   transition: background-color 0.2s ease, border-color 0.2s ease, color 0.2s ease;