Interrupt stalled downloads on Windows too

shutdown() from another thread doesn't wake a blocked recv on Windows, so
quitting mid-download waited out the 4 s deadline there (CI's Windows
server tests). Close the handle as well, detached first so the worker's own
close can't hit a reused handle.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/frame_webinstall.py

@@ -373,6 +373,14 @@ def abort(conn):
             sock.shutdown(socket.SHUT_RDWR)
         except OSError:
             pass
+        if os.name == "nt":
+            # Windows doesn't wake a recv blocked in another thread on shutdown(),
+            # only when the handle closes. detach() first, so the owner's later
+            # close() can't close a handle the OS has since given to someone else.
+            try:
+                socket.close(sock.detach())
+            except OSError:
+                pass
 
 
 def download(p, dest_dir, progress=None, cancelled=None, connected=None):