Make web suggestions generate on demand

[edwin-zvs]

Summary

• make the Web UI suggestion control open History plus Generate instead of immediately calling session.suggest
• expose explicit Regenerate when a hand is cached
• cover the on-demand menu contract in the Web UI smoke test

Verification

• cargo build --workspace
• cargo test -p construct-e2e --test web_smoke --no-run
• cargo test -p construct-e2e --test web_smoke web_client_loads_and_websocket_connects -- --nocapture