Why is this Change Important & Necessary?

Previously, warnings for oversized or partially skipped Jupyter notebooks were
injected as markdown cells inside the rendered notebook itself. This was
inconsistent with Submitty's existing UI patterns for displaying messages, and
made the warnings easy to miss or confuse with actual notebook content.

Closes #11928

What is the New Behavior?

• NotebookUtils::jupyterToSubmittyNotebook() now returns a structured array
  with keys: cells, size_exceeded, skipped_content_count, skipped_output_count
  instead of injecting warning markdown cells directly.
• MiscController.php unpacks these flags and passes them to the Twig template.
• Notebook.twig now displays a red alert banner above the notebook when the
  file exceeds 10MB, and a yellow alert banner when any content/outputs were
  skipped — using Submitty's existing alert styles.

No functional change — same warnings are shown, just displayed outside the
notebook in a cleaner, consistent way.

What steps should a reviewer take to reproduce or test the bug or new feature?

1. Upload a Jupyter notebook (.ipynb) with large image attachments or
   truncated outputs to a gradeable
2. Open the submission in the grading interface
3. Before fix: warning appears as a markdown cell inside the notebook body
4. After fix: warning appears as a colored alert banner above the notebook

To test size exceeded case:

1. Upload a .ipynb file larger than 10MB
2. Open in grading interface
3. Before fix: empty notebook with a markdown error cell
4. After fix: red alert banner above empty notebook

Automated Testing & Documentation

This is a UI consistency improvement with no functional behavior change.
No documentation update required on submitty.org.

Other information

• Not a breaking change
• No migrations required
• No security concerns
• jupyterToSubmittyNotebook() return type changed from flat array to
  structured array — only called in one place (MiscController.php) so
  no other files affected