fix: preserve custom nodes i18n data when locales are lazily loaded (#7214)

## Summary
Custom nodes can provide localized translations via their locales
folder.
After the switch to lazy-loading locales (only English pre-loaded),
custom node i18n data was being lost because:
1. loadCustomNodesI18n() merges data before locale is loaded
2. loadLocale() uses setLocaleMessage() which overwrites everything

Solution: Store custom nodes i18n data and re-merge it after each
lazy-loaded locale completes loading.

- Add mergeCustomNodesI18n() function to store and merge custom data
- Modify loadLocale() to re-merge stored data after loading
- Add unit tests for the i18n merging behavior

fix https://github.com/Comfy-Org/ComfyUI_frontend/issues/7025

## Screenshots (if applicable)
<img width="706" height="1335" alt="image"
src="https://github.com/user-attachments/assets/41e4ba2c-b4c0-4d0d-a104-1ede8939ff92"
/>
<img width="672" height="1319" alt="image"
src="https://github.com/user-attachments/assets/6c3e55e5-20d2-4093-a86a-7496db3dfe94"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7214-fix-preserve-custom-nodes-i18n-data-when-locales-are-lazily-loaded-2c26d73d3650812fab31edd71cf51a19)
by [Unito](https://www.unito.io)

src/components/graph/GraphCanvas.vue

@@ -132,7 +132,7 @@ import { useCopy } from '@/composables/useCopy'
 import { useGlobalLitegraph } from '@/composables/useGlobalLitegraph'
 import { usePaste } from '@/composables/usePaste'
 import { useVueFeatureFlags } from '@/composables/useVueFeatureFlags'
-import { i18n, t } from '@/i18n'
+import { mergeCustomNodesI18n, t } from '@/i18n'
 import { LiteGraph } from '@/lib/litegraph/src/litegraph'
 import { useLitegraphSettings } from '@/platform/settings/composables/useLitegraphSettings'
 import { CORE_SETTINGS } from '@/platform/settings/constants/coreSettings'
@@ -391,9 +391,7 @@ useEventListener(
 const loadCustomNodesI18n = async () => {
   try {
     const i18nData = await api.getCustomNodesI18n()
-    Object.entries(i18nData).forEach(([locale, message]) => {
-      i18n.global.mergeLocaleMessage(locale, message)
-    })
+    mergeCustomNodesI18n(i18nData)
   } catch (error) {
     console.error('Failed to load custom nodes i18n', error)
   }