feat(win): W2/UX19 persist window placement, W10 taskbar flash; tick W11

- W2/UX19: windowState.ts persists the window's normal bounds + maximized flag to
  <userData>/window-state.json (no new dep); createWindow restores from it with a
  screen.getAllDisplays() visibility guard so it never reopens off a disconnected
  monitor. Debounced save on resize/move + save on close (which may hide to tray).
- W10: notify() flashFrame(true)s the taskbar when a completion/failure lands while
  the window is unfocused/minimized (Windows clears the flash on focus).
- W11: already implemented (badge.ts overlay dot + setOverlayIcon) — ticked.

typecheck + 268 tests + eslint + prettier green. (Window restore + flash want a
live quit/relaunch confirm.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/download.ts

@@ -106,6 +106,11 @@ function notify(wc: WebContents, title: string, body: string, incognito = false)
     }
   })
   n.show()
+  // W10: flash the taskbar button for attention when the event happened in the
+  // background (window unfocused/minimized/hidden). Windows stops the flash on its
+  // own once the window gets focus, so there's no explicit stop to manage.
+  const flashWin = BrowserWindow.fromWebContents(wc)
+  if (flashWin && !flashWin.isDestroyed() && !flashWin.isFocused()) flashWin.flashFrame(true)
 }
 
 function logFailure(opts: StartDownloadOptions, title: string | undefined, error: string): void {