Report a missing browser launcher instead of faking success

`open_in_browser` gated its xdg-open path on `target_os = "linux"`, which is
false on Android — that is its own target_os. Android therefore took the
fallback arm, which discarded the URL and returned `Ok(())`.

Login Flow v2 cannot complete without a browser: the user approves the
sign-in there and `auth::poll` waits for that approval. Claiming success meant
the UI showed "Approve the sign-in in your browser" with no browser open, the
poll waited for an approval that could never arrive, and the worker eventually
dropped its channel — surfacing as "sign-in failed unexpectedly", which
pointed at the network rather than at the real cause. The server had in fact
been contacted successfully.

Gate on `unix && !android && !macos` so the arm matches what xdg-open actually
implies, and return Unsupported from the fallback. The caller treats it as
terminal rather than swallowing it with `let _ =`.

Android gets a real Intent-based launcher in the next commit; until then the
failure is at least honest about what happened.

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

ui/dr-ui/src/launch_ui.rs

@@ -290,7 +290,16 @@ fn spawn_login(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>, server:
             };
 
             // Open the system browser, never an embedded webview (FR-NC-1).
-            let _ = open_in_browser(&flow.login_url);
+            //
+            // Failing here is terminal: the poll below waits for an approval
+            // that only the browser can give, so carrying on would hang until
+            // the flow expired and then report nothing useful.
+            if let Err(e) = open_in_browser(&flow.login_url) {
+                let _ = tx.send(LoginMessage::Failed(format!(
+                    "could not open a browser to approve the sign-in: {e}"
+                )));
+                return;
+            }
             let _ = tx.send(LoginMessage::AwaitingApproval(flow.login_url.clone()));
 
             match auth::poll(&client, &flow).await {
@@ -526,8 +535,16 @@ async fn fetch_user_id(
 }
 
 /// Open a URL in the system browser.
+///
+/// Login Flow v2 cannot complete without this: the user approves the sign-in
+/// in a browser and the poll below waits for that approval. So a platform with
+/// no way to open one must say so rather than return `Ok(())` — reporting
+/// success here strands the flow on "Approve the sign-in in your browser" with
+/// no browser and no explanation.
 fn open_in_browser(url: &str) -> std::io::Result<()> {
-    #[cfg(target_os = "linux")]
+    // Not `target_os = "linux"`: Android is its own target_os, and reached this
+    // arm's `Ok(())` fallback, so the browser silently never opened.
+    #[cfg(all(unix, not(target_os = "android"), not(target_os = "macos")))]
     {
         std::process::Command::new("xdg-open")
             .arg(url)
@@ -536,9 +553,12 @@ fn open_in_browser(url: &str) -> std::io::Result<()> {
             .spawn()
             .map(|_| ())
     }
-    #[cfg(not(target_os = "linux"))]
+    #[cfg(not(all(unix, not(target_os = "android"), not(target_os = "macos"))))]
     {
         let _ = url;
-        Ok(())
+        Err(std::io::Error::new(
+            std::io::ErrorKind::Unsupported,
+            "no browser launcher on this platform",
+        ))
     }
 }