fix: inject text-file content into chat completions messages (#9896)

Non-image/non-audio file attachments (txt, md, csv, json) were being
  stored in the 'files' metadata field but never added to the message
  content array sent to /v1/chat/completions. Images and audio correctly
  received content blocks; files did not.

  Fix: push a text content block into messageContent when textContent is
  present, matching the pattern used for image_url and audio_url.

  Also fixes Home.jsx addFiles which never called file.text() at all,
  meaning files attached on the home screen had empty textContent even
  before reaching useChat.js.

  Note: PDF files use file.text() which returns raw bytes rather than
  parsed text. Proper PDF support would require PDF.js or server-side
  extraction and is not part of this fix.

Signed-off-by: Daniel Liljeberg <damien_@hotmail.com>