fix: gate native window reveal on renderer load

Keep BrowserWindow hidden until both ready-to-show and did-finish-load are complete, preventing the incomplete header from reaching the screen.

Render a static upload-speed baseline beneath the transparent canvas so the zero line exists independently of renderer timers.

Add regression coverage for both reveal event orders, load failures, and the script-independent header baseline.

lib/startup-renderer.js

@@ -15,14 +15,31 @@ function createStartupQuery(config, version) {
 
 function createStartupWindow(BrowserWindow, options) {
   const window = new BrowserWindow({ ...options, show: false });
-  window.once('ready-to-show', () => {
+  let nativePaintReady = false;
+  let rendererLoadFinished = false;
+  let revealed = false;
+  const reveal = () => {
+    if (revealed || !nativePaintReady || !rendererLoadFinished) return;
+    revealed = true;
     window.show();
+  };
+  window.once('ready-to-show', () => {
+    nativePaintReady = true;
+    reveal();
+  });
+  window.webContents.once('did-finish-load', () => {
+    rendererLoadFinished = true;
+    reveal();
   });
 
   return {
     window,
     load(target, onLoadError, options) {
-      return window.loadFile(target, options).catch(onLoadError);
+      return window.loadFile(target, options).catch((error) => {
+        rendererLoadFinished = true;
+        reveal();
+        return onLoadError(error);
+      });
     }
   };
 }