Feat(file): add support of the chunked file upload

[Koc]

🚧 WIP

Closes #3449.

What changed

File question uploads previously used a single multipart POST request per
selection. Files were limited by PHP upload limits and could not be resumed.

Uploads now go through a temporary public upload share and the standard
Nextcloud WebDAV endpoint, the same mechanism used by the Files app:

1. POST .../submissions/files/{questionId}/share creates a temporary folder
   in the form owner's storage and shares it via a create-only (file drop)
   public link.
2. The file is uploaded over public.php/dav using the uploader from
   @nextcloud/files (MKCOL workspace, parallel chunk PUTs, MOVE assemble).
   Chunking requires the dav.public_shares_chunking capability; smaller
   files are sent as a plain PUT.
3. POST .../submissions/files/{questionId}/register binds the uploaded file
   to the form and question and returns uploadedFileId + uploadToken,
   which are submitted as the answer exactly as before.

Behavior changes

• Large files are uploaded in chunks with parallel requests and show
  per-file progress instead of a single "Uploading …" indicator.
• Uploads can be cancelled per file.
• Works for public (anonymous) submissions, since files are uploaded through
  a public share owned by the form owner.
• If the share or WebDAV upload fails, the client falls back to the previous
  multipart endpoint, so instances with disabled/restricted link shares keep
  working.
• Stale upload shares and folders are removed by the existing
  CleanupUploadedFilesJob.

API

Two new endpoints (see docs/API_v3.md / openapi.json):

• POST /api/v3/forms/{formId}/submissions/files/{questionId}/share
• POST /api/v3/forms/{formId}/submissions/files/{questionId}/register

The existing POST .../files/{questionId} multipart endpoint is unchanged
and remains supported.

Dependencies

• @nextcloud/files ^4.1.0-beta.2 (new uploader, public chunking support)
• @nextcloud/vue ^9.12.0

How to test

1. Create a form with a file question and open it as a logged-in user and
   via a public share link.
2. Upload a small file and a large file (> chunk size) and verify progress
   is shown and the submission succeeds.
3. Upload two files with the same name (conflict rename) and cancel an
   upload in progress.
4. Disable link sharing (or enforce link passwords) and verify the
   multipart fallback still works.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI (N/A)