Training-metrics sparkline column on the session list

## Proposal (Phase 3 of the training-metrics series)

Surface the training metric (from Phase 1 log parsing / Phase 2 metrics.jsonl) as a **sparkline column** on the session list, so users can scan the health of all running trainings at a glance.

- Sparkline renders the recent loss trend per RUNNING session; empty for sessions with no metric source.
- Polling is constrained: only rows currently visible in the viewport, shared 10–30s interval, backoff when the tab is hidden.
- Reuse the Phase 1 parsing/normalization layer; no new data path.

## Acceptance criteria

- Session list shows live sparklines for visible RUNNING training sessions without noticeable scroll/render jank.
- No log fetches happen for off-screen rows or non-RUNNING sessions.
- Column is hideable via the existing column-setting mechanism.

JIRA Issue: FR-3650

Start at the session-list implementation and trace the Phase 1 log parsing and Phase 2 metrics.jsonl normalization layer, then inspect the existing column-setting mechanism. Verify the result against the acceptance criteria: visible RUNNING sessions show smooth live sparklines, off-screen and non-RUNNING rows are not fetched, polling backs off when hidden, and the column remains hideable.

Written by the indexing model from the issue text.