Add nameplate target arrow color option

[liamcooper]

Summary

• Adds a targetArrowColor setting (default white) for nameplate target arrows on enemy and friendly plates.
• Adds an inline color swatch on the Display page next to Show Arrows on Target, matching other color controls.
• Defines ns.ApplyTargetArrowColor in the friendly nameplates file to avoid Lua 5.1 local-limit errors in the main nameplates module.

Test plan

• Enable Show Arrows on Target on enemy nameplates; confirm arrows appear on your target.
• Change arrow color via the swatch; confirm live update on nameplates and in the options preview.
• Toggle arrows off; confirm the color swatch is dimmed and non-interactive.
• Adjust arrow scale via the cog; confirm size still works with the new color.
• /reload and confirm the chosen color persists.
• With friendly nameplates enabled, confirm target arrows tint correctly on friendly targets.