fix: distinguish attachment upload and processing states

[chenyme]

Summary

Fix misleading attachment status feedback in the chat composer.

Document attachments previously reused the primary upload spinner while queued for extraction or vectorization. This made successfully uploaded TXT and PDF files appear to remain stuck in the upload stage.

• Preserve the file-type icon after the upload request completes.
• Display queued extraction and vectorization progress with a compact secondary spinner and explicit status text.
• Reserve the primary spinner for active file transfers.
• Use the shared isFileProcessing state resolver instead of inferring processing state from presentation tones.
• Localize the upload status in English and Chinese.
• Preserve existing upload polling file processing and attachment lifecycle behavior.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• pnpm check
• pnpm build
• git diff --check

Screenshots, API examples, or logs

The reported screenshot demonstrates that uploaded document attachments retained the same primary spinner used during file transfer.

No API contract or backend behavior changed.

Configuration, migration, and compatibility notes

• No configuration changes.
• No API contract changes.
• No database migrations.
• No deployment changes.
• Existing attachment processing and status polling remain backward compatible.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including file processing and attachment lifecycle behavior.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.