Use health bar border size for cast bars

[DoGi114]

Use health bar border size for cast bars

What does this PR do?

Makes unit-frame cast bars use the configured health-bar border size instead of a hard-coded value.

The existing default of 1 is retained when no border size is configured.

How was it tested?

Tested on my local branch with the stable version of the game ( _retail )

Screenshots

Checklist

• New settings default OFF (no behavior change without opt-in)
  Hard to make it OFF by default since its just inheritance ( which in my opinion should work like that from the begining.
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations)
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames
• Tested in-game, works on live retail; no load errors on the 12.1 PTR client

[DoGi114]

@EllesmereGaming
Hi! When you have a chance, could you review this small cast-bar border-size change? It makes the cast bar use the unit frame’s configured borderSize instead of a hard-coded value. Thanks!
Hope to see it in the next release <3