feat(conversation): prototype infinite-query pagination for classic history

[RonenMars]

Implements step 1 of ADR 0001 (added in this PR: docs/adr/0001-hub-data-layer-lazy-pagination.md).

Why

The Hub eager-drains every page of every server up front (useEagerConversations), which is the source of the progress churn and the "syncing" re-render loop that #563 / #566 pace and guard.
The finest fix is to not drain — paginate lazily.
The infrastructure already exists but was never wired: useConversations is a useInfiniteQuery with no callers, and ConversationList already supports onEndReached / hasNextPage / isFetchingNextPage.

What this prototype does

• Feeds the Classic History ConversationList from the infinite useConversations with a real onEndReached → fetchNextPage, replacing the eager-drained array and the stubbed pagination props (onEndReached={() => {}}, isFetchingNextPage={false}).
• Adds an { enabled } option to useConversations so it only fetches when the History tab is visible.
• Adds a module-level EMPTY_CONVERSATIONS stable reference so the list doesn't see a fresh [] each render while the query is disabled or loading.

Scope / caveats (it's a prototype)

• The eager path still backs the tree/hub/classic-sessions surfaces, so the eager and infinite hooks coexist for now — the documented interim state in the ADR, not the destination.
• On-device verification (scroll-to-load, render-count check with the documented probes) is a follow-up; this PR is compile/lint/type-clean but not yet measured on device.
• Migrating the remaining surfaces off the eager drain, the conversation cache-patch, and colocating status/progress subscriptions are the later ADR steps.

Verification

ESLint clean on the changed files; tsc reports no new errors (the repo-wide router.push typed-routes errors are pre-existing).
The useConversations signature change is backward-compatible (new optional 3rd arg), so existing behavior is unchanged.