feat(download): support ModelScope repositories in the URL downloader

ModelScope became a linkable source, but downloading from it was impossible:
the URL picker only recognised huggingface.co, the file listing hit a
huggingface-only endpoint, the resolve URL was hardcoded, and the default
path template always wrote into a `huggingface/` directory.

Move the download knowledge into the providers so the handlers stay generic:

- `ModelSource` gains `list_files()`, `file_download_url()`,
  `default_revision` and `default_subdir`. `HuggingFaceSource` keeps the Hub
  tree API (`/api/models/{id}/tree/{rev}`, LFS-aware sizes, `main`).
  `ModelScopeSource` uses `/api/v1/models/{id}/repo/files?Revision=master`
  — which reports real byte sizes for LFS files, so no HEAD probe is needed,
  and which only accepts `master` (an HF-imported repo still 404s on `main`)
  — and downloads through `/models/{id}/resolve/{rev}/{path}`. That URL
  redirects to a CDN target carrying a time-limited `auth_key`, so it is
  rebuilt on every request and never cached, which is also what keeps
  resumable Range requests working.
- `hf_handlers.py`/`HfHandler` become `model_source_handlers.py`/
  `ModelSourceHandler` with `list_model_source_files` and
  `download_model_source`. New routes `/api/lm/model-source-files` and
  `/api/lm/download-model-source`; the old `/api/lm/hf-repo-files` and
  `/api/lm/download-hf-model` paths stay as aliases, and a payload without
  `platform` still means Hugging Face, so existing callers are unaffected.
- A downloaded sidecar now records `source_platform` + `source_url` (with the
  `hf_url` alias only for Hugging Face) instead of always writing `hf_url`,
  and `use_default_paths` files ModelScope downloads under
  `modelscope/<owner>/<repo>`. The now-unused shared HF aiohttp session and
  its shutdown hook are gone; providers open short-lived sessions.
- Frontend: `detectUrlType` returns the platform-neutral
  `model-source-repo` / `model-source-file` plus an explicit `platform`, the
  DownloadManager's `hf*` state and methods are renamed to `source*`, every
  `source === 'huggingface'` check becomes `isExternalModelSource()`, and
  batch groups are keyed by `platform:repo` so the same `owner/name` on two
  sites renders as two groups. A bare `owner/name` still means Hugging Face.
- `is_valid_source_id()` centralises repo-id validation (exactly
  `owner/name`, no traversal, no leading dot). This also fixes the old HF
  download check that rejected any dot in the name, i.e. legitimate repos
  such as `black-forest-labs/FLUX.1-dev`.

Verified against the live APIs: the example repo lists 8 weight files with
correct sizes, and a ranged GET of the built resolve URL returns 206 after
following the redirect to the CDN. Backend 2853 passed; frontend 1143 JS +
91 Vue passed. The nine locales carry the refreshed download copy in the
next commit.

tests/frontend/managers/downloadManager.batchSummary.test.js

@@ -26,6 +26,7 @@ const {
       },
     },
     downloadModel: vi.fn(),
+    downloadModelSource: vi.fn(),
     downloadHfModel: vi.fn(),
     cancelDownload: vi.fn(),
     getPageState: vi.fn(() => ({})),
@@ -158,7 +159,7 @@ describe('DownloadManager batch download summary flow', () => {
     // Reset the shared mocks so mockResolvedValueOnce queues and call
     // history never leak between tests.
     mockApiClient.downloadModel.mockReset();
-    mockApiClient.downloadHfModel.mockReset();
+    mockApiClient.downloadModelSource.mockReset();
     mockApiClient.cancelDownload.mockReset();
     showToastMock.mockClear();
     showDownloadBatchSummaryMock.mockClear();
@@ -406,14 +407,15 @@ describe('DownloadManager batch download summary flow', () => {
     expect(showToastMock).toHaveBeenCalledWith('toast.loras.downloadCompleted', {}, 'success');
   });
 
-  it('shows a summary for HF partial failure and retries only the failed files', async () => {
-    manager.hfRepoId = 'user/repo';
-    manager.hfSelectedFiles = ['a.safetensors', 'b.safetensors'];
-    mockApiClient.downloadHfModel
+  it('shows a summary for external repo partial failure and retries only the failed files', async () => {
+    manager.sourcePlatform = 'huggingface';
+    manager.sourceRepoId = 'user/repo';
+    manager.sourceSelectedFiles = ['a.safetensors', 'b.safetensors'];
+    mockApiClient.downloadModelSource
       .mockResolvedValueOnce({ success: true })
       .mockResolvedValueOnce({ success: false, error: 'denied' });
 
-    const result = await manager._downloadHfSingle({ modelRoot: '/m', useDefaultPaths: true });
+    const result = await manager._downloadExternalRepoFiles({ modelRoot: '/m', useDefaultPaths: true });
 
     expect(result).toBe(false);
     expect(showDownloadBatchSummaryMock).toHaveBeenCalledTimes(1);
@@ -428,7 +430,9 @@ describe('DownloadManager batch download summary flow', () => {
 
     await summary.onRetry();
 
-    expect(mockApiClient.downloadHfModel).toHaveBeenCalledTimes(3);
-    expect(mockApiClient.downloadHfModel.mock.calls[2][0].filename).toBe('b.safetensors');
+    expect(mockApiClient.downloadModelSource).toHaveBeenCalledTimes(3);
+    const retryArgs = mockApiClient.downloadModelSource.mock.calls[2][0];
+    expect(retryArgs.filename).toBe('b.safetensors');
+    expect(retryArgs.platform).toBe('huggingface');
   });
 });