Fix Sta mode 100% CPU: skip checkerboard, skip idle repaints

- Replace pixel-by-pixel checkerboard with solid bg in Sta mode
- Animation timer skips repaint in static mode (no animation)
- Refresh timer only repaints Sta when source image actually changes

aniphallow.lua

@@ -1425,12 +1425,9 @@ openPreviewWindow = function()
                     local sy = S.previewStaScrollY
                     local sw = S.sourceImage.width * z
                     local sh = S.sourceImage.height * z
-                    if S.useBgColor then
-                        gc.color = S.bgColor
+                    -- Use solid bg instead of checkerboard for performance
+                    gc.color = S.useBgColor and S.bgColor or Color(50, 50, 50)
                     gc:fillRect(Rectangle(sx, sy, sw, sh))
-                    else
-                        drawCheckerboard(gc, sw, sh, z, sx, sy)
-                    end
                     gc:drawImage(S.sourceImage,
                         Rectangle(0, 0, S.sourceImage.width, S.sourceImage.height),
                         Rectangle(sx, sy, sw, sh))
@@ -1581,18 +1578,28 @@ openPreviewWindow = function()
     previewTimer = Timer{
         interval = S.animSpeed / 1000.0,
         ontick = function()
+            if S.previewMode ~= "static" then
                 pvAnimFrame.value = pvAnimFrame.value + 1
                 pcall(function() previewDlg:repaint() end)
             end
+        end
     }
     previewTimer:start()
 
     pvRefreshTimer = Timer{
         interval = 0.5,
         ontick = function()
+            local prevImg = S.sourceImage
             refreshSource()
             refreshLockFile()
+            -- In static mode, only repaint if source image changed
+            if S.previewMode == "static" then
+                if S.sourceImage ~= prevImg then
                     pcall(function() previewDlg:repaint() end)
+                end
+            else
+                pcall(function() previewDlg:repaint() end)
+            end
 
             -- Auto-detect file change
             local currentFile = app.sprite and app.sprite.filename or ""