Use Gradient AI to automatically generate summaries (TL;DRs) for notes, to help users quickly absorb content. Could be done on save or on demand.

Acceptance Criteria:

• When a note is created or edited, add a button to trigger summary generation.
• UI shows the summary/TL;DR prominently (e.g. at top of the note).
• Users can refresh/regenerate summary.
• Allow disabling summary per note or globally.