feat(castbars): add border customization for unit frame cast bars

[Ricoder92]

Summary

Improves cast bar borders and icon layout across Resource Bars and Unit Frames.

Resource player cast bar

• Fixed the visible gap between the fill and border (if used shadow border texture).
• Added detached icons with a configurable 5–20 spacing.
• Detaching the icon shortens the bar without shifting the overall footprint.

Unit frame cast bars

• Added matching border controls for player, target, focus, and boss cast bars: style, size, color, opacity, offsets, and layer placement.
• Unified the icon and border logic while keeping each unit’s settings independent.
• Added separate borders for detached icons and a shared border for joined icons.
• Matched detached icon backgrounds to the bar’s background color and opacity.
• Kept borders and icon backgrounds visible while idle when the cast bar remains displayed.
• Updated previews and translations across all supported locales, reusing existing locale keys.

Compatibility and safety

• Preserved existing profile layouts without automatic migration writes.
• Kept implementation within existing files.
• Appearance calculations use saved settings rather than secret cast values.
• Live layout changes requested during combat are deferred until combat ends.

Checklist

• New optional behavior defaults OFF; Border Style defaults to Solid, preserving the existing appearance.
• No new polling hooks or update loops.
• Border regions are created lazily and reused; no new per-frame appearance updates.
• No new writes onto Blizzard-owned frames; appearance state and regions belong to the addon.
• Static checks completed.
• Tested in-game on live, including idle visibility, combat transitions, and secret/taint behavior.