Load the Sessions TUI progressively instead of fetching all pages first

[JAORMX]

Part of #583. Depends on work-bounded server pagination.

Problem

The TUI calls every inventory page before rendering anything, leaving operators on an opaque loading… screen for minutes.

Acceptance criteria

• Render the first page before later pages are requested.
• Distinguish initial loading, loading more, stale cursor/restart, retryable failure, cancellation, and completion.
• A later-page failure preserves already loaded rows and offers retry.
• Retry and generation restart cannot duplicate rows.
• Selection, active tab, query, and scroll remain stable as rows arrive.
• Cancellation stops future requests without discarding loaded data.
• Progress is meaningful without inventing precision the server did not supply.
• Startup sessions and interactive /sessions use the same path and are covered end-to-end.