feat(audit): Batch 20 — Windows/main-process (UI23, W12, W15, L131)

UI23: cookie sign-in + PO-token windows open with the app's resolved
theme background (passed from the IPC layer), no more white flash.
W12: multi-resolution fallback tray icon (16/24/32/48 via
addRepresentation dataURL, embedded in trayFallbackIcons.ts); decode +
scale factors verified in an Electron probe.
W15: hardwareAcceleration setting (default false = software rendering);
index.ts gates disableHardwareAcceleration on it after the portable
redirect; Appearance card switch with restart note; boot path verified.
L131: resolved by-design — clearing the queue acknowledges failures;
flashFrame (W10) covers attention, Diagnostics persists the record.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/cookies.ts

@@ -216,7 +216,10 @@ function exportAndResolve(): void {
  */
 export function openCookieLoginWindow(
   url: string,
-  parent?: BrowserWindow
+  parent?: BrowserWindow,
+  /** app-theme window background (UI23) — supplied by the IPC layer so the
+   *  sign-in chrome matches the app's Light/Dark instead of flashing white. */
+  backgroundColor?: string
 ): Promise<CookiesLoginResult> {
   return new Promise((resolve) => {
     let validUrl: string
@@ -241,6 +244,9 @@ export function openCookieLoginWindow(
         width: 480,
         height: 720,
         title: 'Sign in — AeroFetch',
+        // Match the app's resolved theme (UI23) so the window doesn't flash a
+        // default-white background before the remote page paints.
+        backgroundColor,
         autoHideMenuBar: true,
         // Group under the app window instead of taking its own taskbar button
         // (W6); a child window stays above its parent without a modal block.