fix: guard InterfaceOptionsFrame HookScript, defer TankMode class resolve

On the CoA client the legacy InterfaceOptionsFrame global may be nil (the
reworked Settings panel does not always expose it). The unguarded
HookScript calls in Castbar.lua and ClassColours.lua then threw on
addon load. Wrap both in 'if InterfaceOptionsFrame then ... end',
mirroring the guard already present around InterfaceOptionsCombatPanel
in Castbar.lua. SetCVars() is still called unconditionally afterwards.

TankMode.lua resolved the player's class at file scope with
select(2, UnitClass('player')), which on CoA can run before
PLAYER_LOGIN — UnitClass returns nil and 'class' stays nil for the
whole session, breaking IsTank/IsHealer/Toggle. Move the assignment
into mod:OnEnable() (where it is already re-assigned) and leave the
file-scope local as nil. Also document that IsTank/IsHealer only know
about the vanilla 3.3.5 classes; CoA custom classes fall through to
DAMAGER until a CoA-aware tank/healer table is available.

Kui_Nameplates/Modules/Castbar.lua

@@ -416,12 +416,14 @@ function mod:OnInitialize()
 			end
 		)
 	end
+	if InterfaceOptionsFrame then
 		InterfaceOptionsFrame:HookScript(
 			"OnHide",
 			function()
 				SetCVars()
 			end
 		)
+	end
 
 	SetCVars()
 end

Kui_Nameplates/Modules/ClassColours.lua

@@ -106,7 +106,9 @@ function mod:OnInitialize()
 			end
 		end)
 	end
+	if InterfaceOptionsFrame then
 		InterfaceOptionsFrame:HookScript("OnHide", function() SetCVars() end)
+	end
 	SetCVars()
 end
 function mod:OnEnable()

Kui_Nameplates/Modules/TankMode.lua

@@ -8,7 +8,10 @@ local addon = LibStub("AceAddon-3.0"):GetAddon("KuiNameplates")
 local mod = addon:NewModule("TankMode", addon.Prototype, "AceEvent-3.0")
 local L = LibStub("AceLocale-3.0"):GetLocale("KuiNameplates")
 
-local class, tankmode = select(2, UnitClass("player")), nil
+-- `class` is resolved in mod:OnEnable() rather than at file scope: on the CoA
+-- client this file can be parsed before PLAYER_LOGIN, when UnitClass("player")
+-- returns nil and would otherwise leave `class` permanently nil.
+local class, tankmode = nil, nil
 
 local profile_tankmode
 
@@ -137,6 +140,10 @@ do
 		return tankTalents >= 3
 	end
 
+	-- NOTE: only vanilla 3.3.5 classes are matched below. CoA custom classes
+	-- will fall through to DAMAGER in mod:Update(). If a CoA-specific tank /
+	-- healer class table becomes available we should consult it here instead
+	-- of (or in addition to) the hard-coded class strings.
 	local function IsTank()
 		return (class == "WARRIOR" and select(3, GetTalentTabInfo(3)) >= 51) or
 			(class == "DEATHKNIGHT" and IsDeathKnightTank()) or
@@ -144,6 +151,7 @@ do
 			(class == "DRUID" and select(3, GetTalentTabInfo(2)) >= 51 and IsDruidTank())
 	end
 
+	-- See IsTank() note: CoA custom healer classes fall through to DAMAGER.
 	local function IsHealer()
 		return (class == "PALADIN" and select(3, GetTalentTabInfo(1)) >= 51) or
 			(class == "SHAMAN" and select(3, GetTalentTabInfo(3)) >= 51) or