feat: add Update ComfyUI option to Help Center for non-desktop environments (#7578)

## Summary

- Adds "Update ComfyUI" menu item to Help Center for portable/localhost
environments
- Wires existing `/v2/manager/queue/update_comfyui` endpoint to the
frontend
- Only visible in non-desktop, non-cloud distributions (where Electron
update mechanism isn't available)

## Changes

1. **Service layer**: Added `updateComfyUI()` method to
`comfyManagerService.ts`
2. **UI**: Added menu item with download icon to
`HelpCenterMenuContent.vue`
3. **i18n**: Added translation key for the new menu item

## Context

The new Manager UI (v4) lost the ability to update ComfyUI core in
non-desktop environments. This restores that functionality by
integrating the existing manager endpoint into the Help Center menu.

## Test plan

- [ ] Verify menu item appears in portable/localhost environments
- [ ] Verify menu item does NOT appear in desktop (Electron)
environments
- [ ] Verify menu item does NOT appear in cloud environments
- [ ] Test clicking the menu item triggers update and reboot

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7578-feat-add-Update-ComfyUI-option-to-Help-Center-for-non-desktop-environments-2cc6d73d3650811e9e4fe55515f50333)
by [Unito](https://www.unito.io)

tests-ui/tests/store/comfyManagerStore.test.ts

@@ -95,6 +95,7 @@ describe('useComfyManagerStore', () => {
       disablePack: vi.fn().mockResolvedValue(null),
       updatePack: vi.fn().mockResolvedValue(null),
       updateAllPacks: vi.fn().mockResolvedValue(null),
+      updateComfyUI: vi.fn().mockResolvedValue(null),
       rebootComfyUI: vi.fn().mockResolvedValue(null),
       isLegacyManagerUI: vi.fn().mockResolvedValue(false)
     }