feat(connectors): add Azure Blob Storage data source connector (#15466)

### What problem does this PR solve?

Closes #15465.

RAGFlow supports S3, Google Cloud Storage, R2, and OCI as data sources
but not Azure Blob Storage, leaving Azure users without a way to index
container objects into a knowledge base. This adds a first-class Azure
Blob Storage data-source connector — distinct from RAGFlow's existing
Azure storage *backends* (`rag/utils/azure_sas_conn.py`,
`rag/utils/azure_spn_conn.py`) which store RAGFlow's own files.

**Highlights**
- `common/data_source/azure_blob_connector.py`: new `AzureBlobConnector`
(`CheckpointedConnectorWithPermSync` + `SlimConnectorWithPermSync`).
- Uses the existing `azure-storage-blob` dependency (already in
`pyproject.toml`).
  - Three auth modes, tried in order of precedence:
1. **Account key** — `account_name` + `account_key` + `container_name`.
    2. **Connection string** — `connection_string` + `container_name`.
3. **SAS token** — `container_url` + `sas_token` (same shape as
`RAGFlowAzureSasBlob`).
- ETag fingerprint stored per blob in `AzureBlobCheckpoint.etags` —
unchanged blobs (same ETag as last run) are skipped without a download.
Only new/modified blobs are fetched.
  - Optional `prefix` scopes indexing to a virtual folder.
- `validate_connector_settings()` probes `get_container_properties()`
and maps `AuthenticationFailed / 403 / ContainerNotFound` to typed
connector exceptions.
  - Slim-doc IDs are blob names so prune reconciles correctly.
- `common/constants.py`, `common/data_source/config.py`,
`common/data_source/__init__.py`: register `azure_blob` in `FileSource`
/ `DocumentSource` and export `AzureBlobConnector`.
- `rag/svr/sync_data_source.py`: new `AzureBlob(SyncBase)` class routed
through `load_from_checkpoint` (ETag fingerprint owns change-detection)
and added to `func_factory`.
- Frontend:
- `web/src/pages/user-setting/data-source/constant/index.tsx`: new
`DataSourceKey.AZURE_BLOB`, auth-mode selector (account key / connection
string / SAS token), all credential fields, prefix + batch-size,
`syncDeletedFiles` capability, default form values, tile entry with
icon.
- `web/src/locales/{en,zh}.ts`: description + per-field tooltips for all
9 new keys.
- `web/src/assets/svg/data-source/azure-blob.svg`: Azure-branded
stacked-cylinders icon.

**Verification**
- `npm run build` (vite + esbuild) passes (37 s).

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

common/data_source/__init__.py

@@ -36,6 +36,7 @@ from .jira.connector import JiraConnector
 from .sharepoint_connector import SharePointConnector
 from .onedrive_connector import OneDriveConnector
 from .outlook_connector import OutlookConnector
+from .azure_blob_connector import AzureBlobConnector
 from .teams_connector import TeamsConnector
 from .moodle_connector import MoodleConnector
 from .airtable_connector import AirtableConnector
@@ -71,6 +72,7 @@ __all__ = [
     "SharePointConnector",
     "OneDriveConnector",
     "OutlookConnector",
+    "AzureBlobConnector",
     "TeamsConnector",
     "MoodleConnector",
     "BlobType",