viewer: fit shadow frustum to building geometry instead of camera

[wass08]

What does this PR do?

Reworks how the directional-light shadow frustum is framed. It previously followed the camera look-at, which fell apart in practice:

• Zoomed out the fixed ±50 frustum was far too small, so most of the scene got no shadows.
• Zoomed into an empty corner the frustum centred on nothing, leaving the building unshadowed.

Now the frustum is fit to the building geometry, independent of the camera:

• Union the bounds of all registered scene nodes (sceneRegistry.nodes), excluding site nodes so the large ground/site plane doesn't inflate the frustum.
• Fit a bounding sphere → stable focus (center) + radius.
• Size the ortho shadow camera to radius * 1.15 + 3 (the building "and a bit nearby"), park the light just outside the sphere along its theme direction, and bracket near/far around the sphere.
• Bounds are recomputed on a short interval (~0.4s) since they only change while editing; empty scenes fall back to a sensible default region at the origin.

The light direction still comes entirely from the scene theme — only the light's distance and the frustum extents change.

Follow-up to #347.

How to test

1. Run bun dev and open a project with a building.
2. Zoom all the way out — shadows should cover the whole building plus a small surrounding margin (not a tiny patch).
3. Zoom into a corner / pan around — shadows stay anchored to the building rather than tracking the camera.
4. Add, move, or resize geometry — the shadowed region should re-fit within ~0.4s.
5. Switch scene themes (Studio, Paper, Overcast, …) — shadows should still render correctly per theme.

Screenshots / screen recording

N/A — shadow-camera framing change; verified locally with a temporary CameraHelper (removed before this PR) confirming the frustum hugs the building bounds across zoom levels.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch