Add unsupported-version guard popup

v5.0.0 is 12.1-only. If a user force-loads it on an older client (with
"Load out of date AddOns" enabled), show an alert telling them to install
the build that matches their client from CurseForge or Wago. Uses its own
frame so the popup fires even if the main init trips on the wrong client;
only triggers on clients older than 12.1.

Author: mergetest

Core.lua

@@ -3105,6 +3105,37 @@ end
 -- INITIALIZATION
 -- ============================================================
 
+-- ------------------------------------------------------------
+-- Unsupported-version guard
+-- This build is 12.1-only (TOC Interface 120100). WoW hides it as out-of-date
+-- on older clients, but a user with "Load out of date AddOns" enabled would run
+-- it on the wrong version and hit bugs. Warn them to install the matching build.
+-- Uses its OWN frame so the popup fires even if the main init trips on the wrong
+-- client. Only fires for OLDER clients (< 12.1); newer clients are left alone.
+-- ------------------------------------------------------------
+do
+    local MIN_INTERFACE = 120100  -- 12.1.0
+    local clientToc = select(4, GetBuildInfo())
+    if type(clientToc) == "number" and clientToc < MIN_INTERFACE then
+        local guardFrame = CreateFrame("Frame")
+        guardFrame:RegisterEvent("PLAYER_LOGIN")
+        guardFrame:SetScript("OnEvent", function(self)
+            self:UnregisterAllEvents()
+            local L = DF.L
+            if DF.ShowPopupAlert then
+                DF:ShowPopupAlert({
+                    title = L["Unsupported Game Version"],
+                    message = L["DandersFrames doesn't support this version of the game.\n\nThis build is made for World of Warcraft 12.1. Please install the version that matches your client from CurseForge or Wago."],
+                    icon = "Interface\\Icons\\INV_Misc_QuestionMark",
+                    buttons = {
+                        { label = L["OK"], onClick = nil },
+                    },
+                })
+            end
+        end)
+    end
+end
+
 local eventFrame = CreateFrame("Frame")
 eventFrame:RegisterEvent("ADDON_LOADED")
 eventFrame:RegisterEvent("PLAYER_LOGIN")

Locales/enUS.lua

@@ -1147,6 +1147,9 @@ L["These settings are being rebuilt on WoW's new 12.1 aura system and will retur
 L["This is being rebuilt on WoW's new 12.1 aura system and will return in a future update."] = true
 L["WoW's new 12.1 aura system doesn't support these settings, so they can't be provided."] = true
 L["WoW's new 12.1 aura system doesn't support this setting."] = true
+-- 12.1 unsupported-version guard popup (Core.lua)
+L["Unsupported Game Version"] = true
+L["DandersFrames doesn't support this version of the game.\n\nThis build is made for World of Warcraft 12.1. Please install the version that matches your client from CurseForge or Wago."] = true
 L["Cmd + Left Click unavailable on Mac"] = true
 L["Font sizes are not changed. Adjust sizes in each element's page."] = true
 L["Note"] = true