Feature/v2/right rail (#4255)

# Description of Changes

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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)

### 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.

---------

Co-authored-by: ConnorYoh <[email protected]>
Co-authored-by: Anthony Stirling <[email protected]>

frontend/src/styles/theme.css

@@ -106,6 +106,12 @@
   --icon-config-bg: #9CA3AF;
   --icon-config-color: #FFFFFF;
 
+  /* RightRail (light) */
+  --right-rail-bg: #F5F6F8;           /* light background */
+  --right-rail-foreground: #CDD4E1;   /* panel behind custom tool icons */
+  --right-rail-icon: #4B5563;         /* icon color */
+  --right-rail-icon-disabled: #CECECE;/* disabled icon */
+
   /* Colors for tooltips */
   --tooltip-title-bg: #DBEFFF;
   --tooltip-title-color: #31528E;
@@ -234,6 +240,12 @@
   --icon-inactive-bg: #2A2F36;
   --icon-inactive-color: #6E7581;
 
+  /* RightRail (dark) */
+  --right-rail-bg: #1F2329;           /* dark background */
+  --right-rail-foreground: #2A2F36;   /* panel behind custom tool icons */
+  --right-rail-icon: #BCBEBF;         /* icon color */
+  --right-rail-icon-disabled: #43464B;/* disabled icon */
+
   /* Dark mode tooltip colors */
   --tooltip-title-bg: #4B525A;
   --tooltip-title-color: #fff;