Add interrupt cast-bar feedback for target and focus unit frames

[liamcooper]

Summary

Target and focus unit frame cast bars now use the same interrupt feedback as nameplates: dual fill colors based on whether your kick is ready, a grey overlay for uninterruptible casts, and an optional tick showing where the cast will be when your interrupt comes off cooldown.

Kick spell detection and cast-bar tinting are centralized in core EllesmereUI so nameplates and unit frames share one implementation.

Shared changes (EllesmereUI_Kick.lua)

New core module loaded from EllesmereUI.toc:

• kickSpellsByClass — per-class interrupt spell IDs (single source of truth)
• EllesmereUI.GetActiveKickSpell() — resolves the player’s current kick from the spell book
• EllesmereUI.RefreshKickAbility() — refreshes on PLAYER_LOGIN, SPELLS_CHANGED, and reload
• EllesmereUI.ComputeCastBarTint(readyTint, baseTint) — blends interruptible vs on-CD colors using secret-safe APIs (C_Spell.GetSpellCooldownDuration, C_CurveUtil.EvaluateColorValueFromBoolean)

Nameplates no longer duplicate this logic. ns.GetActiveKickSpell and ns.ComputeCastBarTint forward to EllesmereUI, so the cast overlay and on-plate bars behave as before with less code to maintain.

Unit frames

Applies to target and focus only (not player or boss):

• Interruptible cast — castbarFillColor (shown when the cast can be interrupted and your kick is off cooldown)
• Interrupt on CD — castbarInterruptReadyColor (shown while your interrupt is on cooldown)
• Uninterruptible overlay — existing grey shield tint, driven via SetAlphaFromBoolean on notInterruptible
• Kick-ready tick — optional (castbarKickTickEnabled, default on); hidden when the cast is not interruptible or kick is ready

Cast-active tracking registers SPELL_UPDATE_COOLDOWN only while target/focus are casting, matching the nameplate pattern.

Options

On target/focus cast bar settings:

• Two color swatches inline on Show Cast Bar (same tooltips as nameplates: “Interruptible Cast”, “Interrupt on CD”)
• Show Tick at Kick Ready Spot on the row below Show Icon
• Hide When Idle in the left column of its own row (not full width)
• Sync icons to copy cast colors and kick-tick setting between target and focus

Test plan

• /reload on a class with an interrupt (e.g. Demon Hunter)
• Target an enemy casting an interruptible spell: cast bar uses interruptible color when kick is ready, on-CD color when kick is on cooldown
• Confirm kick-ready tick appears only when kick is on CD and cast is interruptible
• Target an uninterruptible cast: grey shield overlay, no tick
• Repeat on focus frame
• Change both swatches in options; confirm target/focus update and sync between them
• Nameplates: confirm cast bar colors and kick tick still work (shared kick module)
• Spec/talent change or /reload: kick spell still resolves correctly