Centralized management of external links (#4471)

Update the desktop guide links to make them platform and locale-aware

Edited by Terry:
Refactor external link management by introducing a centralized
useExternalLink composable with automatic locale and platform detection
for documentation URLs.

- Created useExternalLink composable - A new centralized utility for
managing all external links
- Dynamic docs URL builder (buildDocsUrl) - Automatically constructs
docs.comfy.org URLs with:
  - Locale detection (Chinese vs English)
  - Platform detection (macOS vs Windows for desktop)
  - Flexible path construction with options

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-4471-Add-platform-and-locale-aware-desktop-guide-URL-2346d73d3650815ea4a4dd64be575bbe)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Terry Jia <terryjia88@gmail.com>

tests-ui/tests/components/helpcenter/WhatsNewPopup.test.ts

@@ -12,6 +12,11 @@ vi.mock('vue-i18n', () => ({
   useI18n: vi.fn(() => ({
     locale: { value: 'en' },
     t: vi.fn((key) => key)
+  })),
+  createI18n: vi.fn(() => ({
+    global: {
+      locale: { value: 'en' }
+    }
   }))
 }))