Preserve useful overlapping Web PR contributions in React

[tt-a1i]

Problem

PRs #352, #377, and #358 overlap with the React workbench on main. Their useful work needs to be retained without restoring the legacy frontend or weakening exact-turn cancellation, and original contributors should be credited.

Scope

• Restore feat(web): restore polished workbench UI and conversation rendering #352's missing clipboard fallback in React with honest failure feedback, focus/selection restoration, and cleanup.
• Retain current request deadlines and exact-turn cancellation; add response-body stall and deadline-cleanup regression coverage inspired by fix(web): bound browser request duration #377. feat(web): expose Pi-native turn cancellation #358's unrelated capability-intent changes are outside this scope.
• Credit QuinnWan (@somewan820) and @seekskyworld with source PR links and verified commit author identities.

Validation

Run repository checks and tests, independently review the integration, and preserve attribution in the integration commit. Existing runtime cancellation tests cover stale targeting, repeated cancellation, abort failure, and uncertain settlement.