SceneEditorActivity

Edit a preset's scenes array against an existing preset.

Detailed description

Wraps the spec's `SceneEditorWidget` as a navigable Studio activity. Loads a working copy of a preset by id from the active library on push, lets the operator add / remove / rename scenes and pick per-channel stanza assignments, runs live conflict validation, and writes the edited preset back through `PresetBook::create` (an upsert) when the user saves.

Activity launched with the preset id as its single argument: `activityStack->push("SceneEditorActivity", { presetId })`.

Per-stanza-pair transition durations and other Stanza-level edits stay in `StanzaEditorActivity`; this activity owns scenes only.