Stop the new backend before rolling back a failed WSL toggle

Two follow-ups from the latest review pass:

- main.ts WSL_SET_CONFIG_CHANNEL: when startBackend() succeeded but the
  readiness wait then failed (e.g. a slow first WSL boot exceeding the
  60s timeout), the spawned child kept running under the new mode while
  the on-disk config and renderer state silently reverted, leaving
  runtime and persisted state inconsistent until the next backend exit.
  Stop the new-mode child explicitly, save the previous config, and
  relaunch under it; if the rollback start also fails, fall back to the
  exponential-backoff restart so the loop keeps trying.

- wsl.ts runWslShellScript: the stdin "error" handler and the catch
  around stdin.write/end called settle() without first killing the
  spawned wsl.exe child, so a stdin failure after spawn left the child
  running with no termination path (the timeout that would have killed
  it is cleared by settle()). Kill the child before settle() in those
  paths to match the timeout-handler shape.

Author: Jgratton24

apps/desktop/src/main.ts

@@ -2016,12 +2016,17 @@ function registerIpcHandlers(): void {
         await waitForBackendWindowReady(backendHttpUrl);
         return true;
       } catch (error) {
-        // Roll back so the on-disk config matches the renderer's view; the
-        // exponential-backoff restart will relaunch under the previous
-        // configuration. Without this, the renderer's error toast leaves the
-        // UI showing the old toggle while the next app launch silently honors
-        // the never-confirmed configuration.
+        // Stop whatever started under the new config (it may still be alive
+        // if startBackend succeeded but readiness then timed out), revert
+        // the on-disk config, and relaunch under the previous configuration.
+        // Without the explicit stop, runtime and persisted state can diverge
+        // until the new-mode child exits on its own.
+        await stopBackendAndWaitForExit().catch(() => {});
         saveWslConfig(STATE_DIR, previousConfig);
+        const restarted = await startBackend().catch(() => false);
+        if (!restarted) {
+          scheduleBackendRestart("WSL config rollback");
+        }
         throw error;
       }
     };

apps/desktop/src/wsl.ts

@@ -235,14 +235,19 @@ function runWslShell(
 
     const stdin = child.stdin;
     if (stdin) {
-      stdin.once("error", (error) => settle(127, `\n${error.message}`));
+      stdin.once("error", (error) => {
+        child.kill();
+        settle(127, `\n${error.message}`);
+      });
       try {
         stdin.write(bashScript);
         stdin.end();
       } catch (error) {
+        child.kill();
         settle(127, `\n${error instanceof Error ? error.message : String(error)}`);
       }
     } else {
+      child.kill();
       settle(127, "\n[stdin pipe unavailable]");
     }
   });