Add data upload capability to the web UI

[stalep]

Problem

The h5m web UI has no way to upload performance data. Users must use the CLI (upload command) or the REST API (POST /api/folder/{name}/upload) directly. We should not encourage users to upload using the web UI, but I think they should be able to.

Current state

• Upload endpoint: POST /api/folder/{name}/upload exists and works
• Upload via CLI: The upload command supports uploading JSON files from a directory
• Upload via UI: Missing entirely

Proposed implementation

Upload JSON data

• Add an "Upload" button to the FolderPage header or Data tab
• Options for input:
  • File upload — drag-and-drop or file picker for JSON files
  • Paste JSON — text area for pasting JSON data directly
  • Directory upload — upload multiple files at once (maps to multiple upload calls)
• Show upload progress and completion status
• Refresh the data view after successful upload

Upload status

• After upload, show a brief status indicating success/failure
• Optionally show the processing status (the upload triggers async node evaluation, which may take time for large data with complex node graphs)
• Link to the recalculation status endpoint for progress tracking

UX considerations

• For bulk imports (many files), consider a batch upload mode with progress tracking
• Large JSON files (e.g., rhivos at 14 MB) should show upload progress
• Error handling: display server error messages clearly (e.g., invalid JSON, folder not found)

Related

• FolderService.upload() returns an Upload record with the value ID and a CompletableFuture
• The upload triggers async cascade processing through the node DAG via the work queue
• ProcessingTrackerEntity tracks upload completion for crash recovery