Deprecate the legacy waveai block UI and add a preview for its replacement state (#3122)

- [x] Inspect the current legacy Wave AI block UI and widget config
references
- [x] Restyle the deprecated `waveai` block message to remove the inner
card/icon and position it slightly above center
- [x] Remove `defwidget@ai` from default widgets and delete the related
frontend special-case filtering
- [x] Update the widgets preview mock data and scenarios to reflect the
removed AI default widget
- [x] Re-run targeted validation, review, and security scan

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>

Author: Copilot