fix(bossframes): Fix boss frames borders

[Ricoder92]

Summary

This PR fixes boss frames incorrectly inheriting their border settings from the Focus/Target frame donor instead of using their own Boss Frame border configuration.

Changes

• Boss frames now use their dedicated border settings for:
  • Border style and texture
  • Border size
  • Border color and opacity
  • Border offsets and texture shifts
  • “Show Behind” behavior
• Boss frame and aura previews now accurately match the configured boss borders.
• Boss aura borders now refresh immediately after border setting changes.
• Fixed compatibility with the Boss Frame Hover Borders options:
  • Mouseover uses the configured Hover Border Color.
  • The current boss target uses the configured Target Border Color.
  • Hover takes priority over Target.
  • The default boss border is restored when neither state is active.
• Borders using Show Behind are temporarily raised during Hover/Target states so the highlight remains visible.
• Migrated the legacy Target-of-Target border size override to the new independent border settings while preserving existing user configurations.

Result

Boss frames no longer inherit Focus/Target border settings. They consistently use their own Boss Frame border configuration while correctly supporting mouseover and target border highlights.

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