- sessions.js _formatSourceTag(): return null for unrecognised tags
instead of raw string — prevents legacy 'N/A' values from surfacing
- sessions.js metaBits push: guarded with _stLabel null check so only
known platform labels appear in the session metadata line
- sessions.js [SYSTEM:] title fallback: drop raw s.source_tag middle
term, fall back directly to 'Gateway' for unknown sources
7 new tests in test_issue429.py.
1 updated test in test_sprint40_ui_polish.py (new guarded push pattern).
Closes#429
