Open the sign-in URL on Windows

Login Flow v2 cannot complete without a browser, and the launcher had
a branch for xdg-open, one for Android's Intent, and an honest
Unsupported error for everything else — which on Windows stranded the
flow on "approve the sign-in in your browser". rundll32
url.dll,FileProtocolHandler is ShellExecute on the URL and needs no
crate; chosen over cmd /C start, whose quoting of & in a query string
is a known trap. Not verified: Wine has no browser to open.

ui/dr-ui/src/launch_ui.rs

@@ -816,9 +816,26 @@ fn open_in_browser(url: &str) -> std::io::Result<()> {
     {
         android_open_url(url).map_err(|e| std::io::Error::other(e))
     }
+    // TRACES: FR-PLAT-WIN-2
+    // `ShellExecute` by way of the shell's URL handler, which is what a
+    // double-click on a link does, and needs no crate: `rundll32
+    // url.dll,FileProtocolHandler` has opened the default browser since
+    // Windows 98 and is still what the platform documents for the purpose.
+    // Not `cmd /C start`, whose quoting of `&` in a query string is a
+    // well-known trap.
+    #[cfg(windows)]
+    {
+        std::process::Command::new("rundll32")
+            .args(["url.dll,FileProtocolHandler", url])
+            .stdout(std::process::Stdio::null())
+            .stderr(std::process::Stdio::null())
+            .spawn()
+            .map(|_| ())
+    }
     #[cfg(not(any(
         all(unix, not(target_os = "android"), not(target_os = "macos")),
-        target_os = "android"
+        target_os = "android",
+        windows
     )))]
     {
         let _ = url;