MidnightNameplates Phase 9.3.9 - Preview Layout Boundary Fix

Structural fix:
- The Live Preview now owns a fixed top header region.
- All option pages begin below that boundary.
- General/Health/Rules/etc can no longer overlap the preview panel.
- Added a subtle divider between preview and settings content where supported.
- Preview note moved inside the preview region.
- Nameplate stack moved slightly upward inside the preview to rebalance the composition.
- Aura rows tightened slightly.
- No live nameplate runtime logic changed.

Test:
1. /mnp opens.
2. GENERAL heading must start fully below preview panel.
3. Preview note stays inside preview area.
4. Health/Name/Target/Rules pages all start at the same safe vertical boundary.
5. Scrollbars still work.
6. No Lua errors.
