[feat] Add model metadata fetching with loading skeleton and gated repo support (#9415)

## Summary
- Fetch file sizes via HEAD requests (HuggingFace) and Civitai API with
caching and request deduplication
- Show skeleton loader while metadata is loading
- Display "Accept terms" link for gated HuggingFace models instead of
download button

## Changes
- **`missingModelsUtils.ts`**: Add `fetchModelMetadata` with Civitai API
support, HuggingFace gated repo detection, in-memory cache, and inflight
request deduplication
- **`MissingModelsContent.vue`**: Add Skeleton loading state, gated
model "Accept terms" link, extract `showSkeleton` helper
- **`missingModelsUtils.test.ts`**: Tests for HEAD/Civitai fetching,
gated repo detection, caching, and deduplication
- **`main.json`**: Add `acceptTerms` i18n key

## Related Issues
https://github.com/Comfy-Org/ComfyUI_frontend/issues/9410
https://github.com/Comfy-Org/ComfyUI_frontend/issues/9412


┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9415-feat-Add-model-metadata-fetching-with-loading-skeleton-and-gated-repo-support-31a6d73d36508127859efa0b3847505e)
by [Unito](https://www.unito.io)

src/components/dialog/content/missingModelsUtils.test.ts

@@ -0,0 +1,142 @@
+import { describe, expect, it, vi, beforeEach } from 'vitest'
+
+import { fetchModelMetadata } from './missingModelsUtils'
+
+const fetchMock = vi.fn()
+vi.stubGlobal('fetch', fetchMock)
+
+vi.mock('@/platform/distribution/types', () => ({ isDesktop: false }))
+vi.mock('@/stores/electronDownloadStore', () => ({}))
+
+let testId = 0
+
+describe('fetchModelMetadata', () => {
+  beforeEach(() => {
+    fetchMock.mockReset()
+    testId++
+  })
+
+  it('fetches file size via HEAD for non-Civitai URLs', async () => {
+    fetchMock.mockResolvedValueOnce({
+      ok: true,
+      headers: new Headers({ 'content-length': '1048576' })
+    })
+
+    const url = `https://huggingface.co/org/model/resolve/main/head-${testId}.safetensors`
+    const metadata = await fetchModelMetadata(url)
+    expect(metadata.fileSize).toBe(1048576)
+    expect(metadata.gatedRepoUrl).toBeNull()
+    expect(fetchMock).toHaveBeenCalledWith(url, { method: 'HEAD' })
+  })
+
+  it('uses Civitai API for Civitai model URLs', async () => {
+    const url = `https://civitai.com/api/download/models/${testId}`
+    fetchMock.mockResolvedValueOnce({
+      ok: true,
+      json: async () => ({
+        files: [{ sizeKB: 1024, downloadUrl: url }]
+      })
+    })
+
+    const metadata = await fetchModelMetadata(url)
+    expect(metadata.fileSize).toBe(1024 * 1024)
+    expect(metadata.gatedRepoUrl).toBeNull()
+    expect(fetchMock).toHaveBeenCalledWith(
+      `https://civitai.com/api/v1/model-versions/${testId}`
+    )
+  })
+
+  it('returns null fileSize when Civitai API fails', async () => {
+    fetchMock.mockResolvedValueOnce({ ok: false })
+
+    const metadata = await fetchModelMetadata(
+      `https://civitai.com/api/download/models/${testId}`
+    )
+    expect(metadata.fileSize).toBeNull()
+    expect(metadata.gatedRepoUrl).toBeNull()
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('returns gatedRepoUrl for gated HuggingFace HEAD requests (403)', async () => {
+    fetchMock.mockResolvedValueOnce({ ok: false, status: 403 })
+
+    const metadata = await fetchModelMetadata(
+      `https://huggingface.co/bfl/FLUX.1/resolve/main/gated-${testId}.safetensors`
+    )
+    expect(metadata.gatedRepoUrl).toBe('https://huggingface.co/bfl/FLUX.1')
+    expect(metadata.fileSize).toBeNull()
+  })
+
+  it('does not treat HuggingFace 404/500 as gated', async () => {
+    fetchMock.mockResolvedValueOnce({ ok: false, status: 404 })
+
+    const metadata = await fetchModelMetadata(
+      `https://huggingface.co/org/model/resolve/main/notfound-${testId}.safetensors`
+    )
+    expect(metadata.gatedRepoUrl).toBeNull()
+    expect(metadata.fileSize).toBeNull()
+  })
+
+  it('returns null for unrecognized Civitai URL patterns', async () => {
+    const url = `https://civitai.com/api/v1/models/${testId}`
+    const metadata = await fetchModelMetadata(url)
+    expect(metadata.fileSize).toBeNull()
+    expect(metadata.gatedRepoUrl).toBeNull()
+    expect(fetchMock).not.toHaveBeenCalled()
+  })
+
+  it('returns cached metadata on second call', async () => {
+    const url = `https://huggingface.co/org/model/resolve/main/cached-${testId}.safetensors`
+
+    fetchMock.mockResolvedValueOnce({
+      ok: true,
+      headers: new Headers({ 'content-length': '500' })
+    })
+
+    const first = await fetchModelMetadata(url)
+    const second = await fetchModelMetadata(url)
+
+    expect(first.fileSize).toBe(500)
+    expect(second.fileSize).toBe(500)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('does not cache incomplete results so retries are possible', async () => {
+    const url = `https://example.com/retry-${testId}.safetensors`
+
+    fetchMock
+      .mockResolvedValueOnce({
+        ok: true,
+        headers: new Headers({})
+      })
+      .mockResolvedValueOnce({
+        ok: true,
+        headers: new Headers({ 'content-length': '1024' })
+      })
+
+    const first = await fetchModelMetadata(url)
+    const second = await fetchModelMetadata(url)
+
+    expect(first.fileSize).toBeNull()
+    expect(second.fileSize).toBe(1024)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+  })
+
+  it('deduplicates concurrent requests for the same URL', async () => {
+    const url = `https://huggingface.co/org/model/resolve/main/dedup-${testId}.safetensors`
+
+    fetchMock.mockResolvedValueOnce({
+      ok: true,
+      headers: new Headers({ 'content-length': '2048' })
+    })
+
+    const [first, second] = await Promise.all([
+      fetchModelMetadata(url),
+      fetchModelMetadata(url)
+    ])
+
+    expect(first.fileSize).toBe(2048)
+    expect(second.fileSize).toBe(2048)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+})