The full rebuild path of _write_session_index scans SESSION_DIR via
glob('*.json') and appends every loaded session to a plain list without
deduplicating by session_id. When old-format session_*.json files coexist
alongside WebUI-format xxx.json files (both sharing the same session_id),
the same session appears multiple times in the index, causing frontend
Vue key collisions and a blank page.
Fix: use dict[session_id -> compact_entry] to naturally deduplicate.
Prefer the entry with the larger message_count when conflicts arise.
wdzhou
16f9887846