fix(chat): Fix inner borders in chat

[Ricoder92]

Summary

Fixes two inner-border rendering issues:

• Fixed the inner border strata so it renders in the correct layer and remains visible above the relevant panel backgrounds.
• Fixed the sidebar divider rendering: the inner border is no longer shown when the sidebar background is disabled, preventing an unnecessary divider and incorrect layout.

Notes

These changes are visual-only and do not change chat frame layout or positioning.

Checklist

• New settings default OFF
• Zero cost while disabled
• Cheap while enabled
• No writes onto Blizzard-owned frame
• Tested in-game, works on live retail; no load errors on the 12.1 PTR client