Wire the `AppealIntakePage.jsx` to make real API calls for case creation (`POST /cases/`) and document uploads (`POST /cases/{id}/documents`). This includes displaying inline per-file upload progress and error handling, while preserving demo mode behind a feature flag.
## Context Wires `AppealIntakePage.jsx` to make real API calls: `POST /cases/` to create the case, then `POST /cases/{id}/documents` for each file, with per-file upload progress displayed inline. The demo mode path is preserved behind a feature flag. ## Requirements * Submission calls `POST /cases/` and receives a real `case_id` from the backend * Each file is streamed to `POST /cases/{id}/documents` with inline progress indicator * Upload errors are shown inline per file (not as a generic modal) * Demo case scenario (used for QA walkthroughs) routes through a feature flag, not the real upload path ## Acceptance criteria - [ ] Successful submission produces a real `case_id` from the backend (not a mock UUID) - [ ] Per-file upload progress is shown during the documents upload - [ ] File-level errors (e.g. unsupported type, size exceeded) are shown inline beside the file - [ ] Demo mode feature flag bypasses the real upload path - [ ] After successful upload, the UI transitions to t