perf(sessions): coalesce eager-fetch progress and memoize list roots

[RonenMars]

Scope (partial improvement — not the full fix)

This PR paces two sources of re-render churn on the Hub during the "Showing cached data — syncing" state, but on-device measurement (see #564 / #565) showed it does not by itself stop the sustained re-render loop.
Treat it as a smaller, complementary improvement to the root-cause fixes in #566, not as "the fix" for the slow syncing state.

What it does

1. Coalesce the eager-fetch progress setState in useEagerSessions and useEagerConversations with useThrottledCallback (250 ms, .flush() on completion so the final counter stays exact).
   This collapses the per-page progress bursts from ~15-20 root re-renders per refresh to a handful, and applies to all three views because the hooks live once at the Hub root.
2. Wrap the four list roots — TreeSessionsList, ProjectHubList, ClassicSessionsList, MergedClassicList — in React.memo so stable-prop drain ticks skip re-running the list for tree, hub, and classic alike.

No behavior change: React.memo only skips a re-render when props are shallow-equal, and every list prop is a fresh ref from useMemo/state.

Why it isn't the whole story

On-device tracing with a per-commit probe found the Hub still re-rendering continuously after this change, driven by two things this PR does not touch:

• The serverFetchStatus store re-rendering subscribers on every HTTP response (serverFetchStatus store re-renders subscribers on every HTTP response (fresh object on unchanged status) #564).
• conversation_updated WS frames re-draining the whole eager conversations query per frame (conversation_updated WS frames trigger a full eager-conversations re-drain per frame (sustained Hub re-render loop) #565).

Both are fixed in #566, which is what actually stops the loop (render rate ~6/sec → ~0.65/sec on-device). This PR remains worthwhile because it bounds the per-page progress bursts that #566 does not address.

Verification

ESLint clean; tsc no new errors (the repo-wide router.push typed-routes errors are pre-existing and unrelated).
Unit: useSession.test 5/5 and ServerRowRefreshing 10/10 pass locally; useConversations.test left to CI.