feat(toolbar): remove the legacy side palette and standardize on the unified top tool

[devmobasa]

Summary

• Make the unified top toolbar the only supported toolbar interface.
• Remove the legacy left-side palette from the built-in, inline, Wayland, and GTK implementations.
• Delete its layout, rendering, surfaces, hit-testing, dragging, events, runtime state, and configurator controls.
• Keep Canvas, Session, Settings, presets, and drawing controls accessible through top-toolbar popovers, islands, and the contextual style pill.

Compatibility

• Preserve retired sidebar settings in authored config.toml files during unrelated saves.
• Report those settings as RetiredSetting instead of generic unknown settings.
• Keep historical side.* item IDs only when they still customize controls now hosted by the unified toolbar.
• Preserve five legacy runtime values as inert passthrough data; they cannot affect or recreate the sidebar.
• Prune obsolete generated runtime order groups on rewrite.
• Document the pre-1.0 Rust configuration API break in the changelog.