The per-session icon picker added more noise than value — rip it out end
to end (sessions.icon column, set_session_icon, the PATCH field, the
picker UI, and the SessionInfo.icon type).
The cross-profile session aggregator now opens each profile's state.db
read-only (mode=ro, no schema init), so listing other profiles on every
sidebar refresh never DDLs or takes a write lock on their live DBs. The
single-profile hot path stays on par with /api/sessions.
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