Feature/onboarding slides (#4955)

# Description of Changes

- Added onboarding slides/upgrade banner conditions for all the
following cases
  - 'licensed'
  - 'no-login-user-under-limit-no-license'
  - 'no-login-admin-under-limit-no-license'
  - 'no-login-user-over-limit-no-license'
  - 'no-login-admin-over-limit-no-license'
  - 'login-user-under-limit-no-license'
  - 'login-admin-under-limit-no-license'
  - 'login-user-over-limit-no-license'
  - 'login-admin-over-limit-no-license';


---

## 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)

### 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)

- [ ] 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: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>
Co-authored-by: Connor Yoh <connor@stirlingpdf.com>

frontend/src/core/constants/downloads.ts

@@ -0,0 +1,10 @@
+// Centralized download URLs for Stirling PDF desktop installers
+export const DOWNLOAD_URLS = {
+  WINDOWS: 'https://files.stirlingpdf.com/win-installer.exe',
+  MAC_APPLE_SILICON: 'https://files.stirlingpdf.com/mac-installer.dmg',
+  MAC_INTEL: 'https://files.stirlingpdf.com/mac-x86_64-installer.dmg',
+  LINUX_DOCS: 'https://docs.stirlingpdf.com/Installation/Unix%20Installation/',
+} as const;
+
+export const DOWNLOAD_BASE_URL = 'https://files.stirlingpdf.com/';
+

frontend/src/core/constants/events.ts

@@ -0,0 +1,27 @@
+import type { LicenseNotice } from '@app/types/types';
+
+export const ONBOARDING_SESSION_BLOCK_KEY = 'stirling-onboarding-session-active';
+export const ONBOARDING_SESSION_EVENT = 'stirling:onboarding-session-started';
+export const SERVER_LICENSE_REQUEST_EVENT = 'stirling:server-license-requested';
+export const UPGRADE_BANNER_TEST_EVENT = 'stirling:upgrade-banner-test';
+export const UPGRADE_BANNER_ALERT_EVENT = 'stirling:upgrade-banner-alert';
+
+export interface ServerLicenseRequestPayload {
+  licenseNotice?: Partial<LicenseNotice>;
+  deferUntilTourComplete?: boolean;
+  selfReportedAdmin?: boolean;
+}
+
+export type UpgradeBannerTestScenario = 'friendly' | 'urgent-admin' | 'urgent-user' | null;
+
+export interface UpgradeBannerTestPayload {
+  scenario: UpgradeBannerTestScenario;
+}
+
+export interface UpgradeBannerAlertPayload {
+  active: boolean;
+  audience?: 'admin' | 'user';
+  totalUsers?: number | null;
+  freeTierLimit?: number;
+}
+