path (#4488)

# Description of Changes

<!--
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.

frontend/src/constants/app.ts

@@ -5,3 +5,19 @@ export const getBaseUrl = (): string => {
   const { config } = useAppConfig();
   return config?.baseUrl || 'https://stirling.com';
 };
+
+// Base path from Vite config - build-time constant, normalized (no trailing slash)
+// When no subpath, use empty string instead of '.' to avoid relative path issues
+export const BASE_PATH = (import.meta.env.BASE_URL || '/').replace(/\/$/, '').replace(/^\.$/, '');
+
+/** For in-app navigations when you must touch window.location (rare). */
+export const withBasePath = (path: string): string => {
+  const clean = path.startsWith('/') ? path : `/${path}`;
+  return `${BASE_PATH}${clean}`;
+};
+
+/** For OAuth (needs absolute URL with scheme+host) */
+export const absoluteWithBasePath = (path: string): string => {
+  const clean = path.startsWith('/') ? path : `/${path}`;
+  return `${window.location.origin}${BASE_PATH}${clean}`;
+};