Add stage completion certificates/badges (Priority 3 #2)

Certificate System:
- JSON certificates stored in .training_state/certificates/
- Includes stage number, name, completion timestamp, wallet name
- Unique certificate_id for each completion
- Version tracking for future updates

New Functions:
- get_stage_name() - Map stage number to name
- generate_certificate() - Create JSON certificate file
- display_badge() - ASCII art badge on completion
- view_certificates() - List and view earned certificates
- export_certificate() - Export certificate to home directory

Integration:
- Certificate generation in run_stage() on success
- Certificate viewing in main menu (option 7)
- Certificate viewing/exporting in playground menu (options 6-7)
- Certificates cleared on training state reset

Updates:
- Added CERT_DIR variable for certificate storage
- Initialize CERT_DIR in main()
- Updated .gitignore for certificates directory
- Menu options increased to accommodate certificate features

Usage:
- Certificates auto-generated on stage completion
- View via menu option 7 or playground option 6
- Export to home directory via playground option 7

.gitignore

@@ -54,6 +54,7 @@ dev/cache/logs/
 # ===================
 scripts/training/.training_progress
 scripts/training/.training_state/
+scripts/training/.training_state/certificates/
 dev/config/
 dev/test-nodes/*/data/
 # Keep coordinator-api data directory (contains application code)