Clear castbar unlock preview when leaving unlock mode

The unlock preview drives the bar via alpha: the OnUpdate forces alpha=1
while the drag handle is shown. On lock, with no active cast (endTime nil)
and no fadeout, the idle branch returned early and left the empty bar
stuck at alpha 1. Reset leftover alpha to 0 in that branch so the preview
clears once the drag handle hides. Fixes #16.

modules/castbar.lua

@@ -241,7 +241,10 @@ pfUI:RegisterModule("castbar", function ()
         return
       end
 
-      if not this.endTime then return end
+      if not this.endTime then
+        if this:GetAlpha() ~= 0 then this:SetAlpha(0) end
+        return
+      end
 
       -- Non-player bars: if the unit disappears (target died / detarget),
       -- drop the bar immediately.