#994 Start experiences directly in chat and simplify the gallery

Clicking an experience now sends its workflow directly to the host conversation instead of opening a generic brief modal. The agent uses existing context and asks only for missing essentials. Both thumbnail and expanded-card clicks share the handoff, with duplicate-click protection and retry feedback.

Also removes the gallery toolbar, reuses playing videos during hover expansion, and improves hover responsiveness. Keyboard focus can expand the preview without interrupting pointer clicks.

Validation: 10 UI unit tests pass; UI ESLint, bundle compilation, and diff checks pass. Pointer and keyboard activation verified in the local preview with a simulated host. Live ChatGPT handoff has not yet been verified.