Scene Navigator empty state doesn't let you create a scene

[stultus]

Area: src/lib/components/SceneNavigator.svelte
Severity: Medium

Problem

When a document has zero scenes, the navigator shows "No scenes yet" but offers no
way to create one. The user has to switch to the Cards view to click the Add
button, which is a surprising detour.

Suggested fix

Add an "Add scene" button / primary CTA inside the empty state that inserts a
blank `scene_heading` at cursor position (or at the end of the doc if the editor
is empty).