feat: scheduled tab — resend, edit, duplicate + UX improvements

- Add Resend button (done/cancelled jobs): inline datetime picker clones job at new time
- Add Cancel & Edit button (pending jobs): loads content into Compose/Groups tab, cancels original
- Add Duplicate button (all jobs): loads content into tab for fresh scheduling, original untouched
- Migrate scheduler storage from localStorage to IndexedDB
- Add inline message preview (icon + truncated content) directly on job cards
- Cards collapse by default; Details ▾/▴ toggles full message + recipient list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Eran