Add SiloHistoryViewer widget that opens in an MDI subwindow when the
user double-clicks the History node in the Silo tree. Displays revision
cards newest-first with revision number, Catppuccin-themed lifecycle
status badges, author, timestamp, and commit comment.
Changes:
- silo_viewers.py: SiloHistoryViewer with revision card layout,
status badge QSS, scroll area, empty-history placeholder
Closes#40
29f4a7b110