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.

renderer/index.html

@@ -55,7 +55,10 @@
     <div class="header-spacer" aria-hidden="true"></div>
     <div class="header-cluster header-utilities">
       <div class="upload-speed-sparkline" id="uploadSpeedSparkline" title="Aktuelle Upload-Geschwindigkeit" aria-label="Aktuelle Upload-Geschwindigkeit">
-        <canvas id="uploadSpeedCanvas" width="232" height="44" aria-hidden="true"></canvas>
+        <span class="upload-speed-chart" aria-hidden="true">
+          <span class="upload-speed-baseline"></span>
+          <canvas id="uploadSpeedCanvas" width="232" height="44"></canvas>
+        </span>
         <strong id="uploadSpeedValue">0 B/s</strong>
       </div>
       <button class="header-update-button" id="headerUpdateBtn" title="Nach Aktualisierungen suchen" aria-label="Nach Aktualisierungen suchen" data-tooltip="Nach Aktualisierungen suchen">

renderer/styles.css

@@ -1960,7 +1960,27 @@ select.hs-input { max-width: none; width: auto; min-width: 140px; }
   transition: opacity .16s ease;
 }
 
+.upload-speed-chart {
+  position: relative;
+  min-width: 0;
+  height: 22px;
+  display: block;
+}
+
+.upload-speed-baseline {
+  position: absolute;
+  left: 0;
+  right: 0;
+  bottom: 2px;
+  height: 2px;
+  border-radius: 999px;
+  background: var(--success);
+  pointer-events: none;
+}
+
 .upload-speed-sparkline canvas {
+  position: relative;
+  z-index: 1;
   width: 100%;
   height: 22px;
   display: block;
@@ -4053,7 +4073,7 @@ input[type="checkbox"] {
     justify-content: flex-end;
   }
 
-  .upload-speed-sparkline canvas {
+  .upload-speed-chart {
     display: none;
   }