MidnightNameplates Phase 9.3.10 - Content Anchor Fix

ROOT CAUSE:
- The preview host was 220px tall and started at -18.
- The settings content still started at -196.
- Therefore the settings canvas physically overlapped the preview area.

FIX:
- Settings content now begins at -292 inside the content panel.
- Top-left and top-right anchors are explicit.
- Bottom anchor remains independent for scrolling.
- Preview note remains directly below preview.
- All pages inherit this same safe boundary automatically.

No preview internals or live nameplate runtime logic changed.

TEST:
1. /mnp opens.
2. GENERAL heading must be fully below the preview and note.
3. Health/Name/Target/Rules pages all start at the same lower boundary.
4. Scrollbars still work with mouse wheel and drag.
5. No Lua errors.
