fix(dev): forward termination signals to Electron child

Spawn the Electron binary directly in dev/preview mode but forward
SIGINT/SIGTERM/SIGHUP from the electron-vite parent to the child,
matching Electron's own CLI wrapper (node_modules/electron/cli.js).

Without this, Ctrl-C sends SIGINT to the foreground process group once:
the Node parent exits immediately (no handler) and the Electron child
gets only a single SIGINT. On macOS, an Electron app whose before-quit
handler calls preventDefault() and later calls app.quit() can stall in
AppKit's event loop in that path. Under a direct 'electron .' launch
the CLI shim forwards SIGINT as well, so Electron gets it twice and
exits cleanly. Matching that behavior here fixes the hang.

Fixes #899

src/electron.ts

@@ -161,5 +161,32 @@ export function startElectron(root: string | undefined): ChildProcess {
   const ps = spawn(electronPath, [entry].concat(args), { stdio: 'inherit' })
   ps.on('close', process.exit)
 
+  forwardTerminationSignals(ps)
+
   return ps
 }
+
+let signalHandlersInstalled = false
+let currentElectronPs: ChildProcess | undefined
+
+// Forward termination signals to the Electron child, matching electron's own
+// CLI wrapper (node_modules/electron/cli.js). Without this, Ctrl-C kills the
+// parent immediately and the Electron child can get stuck on macOS when a
+// before-quit handler calls preventDefault then later app.quit(). See #899.
+function forwardTerminationSignals(ps: ChildProcess): void {
+  currentElectronPs = ps
+  if (signalHandlersInstalled) return
+  signalHandlersInstalled = true
+  for (const signal of ['SIGINT', 'SIGTERM', 'SIGHUP'] as const) {
+    process.on(signal, () => {
+      const target = currentElectronPs
+      if (target && target.exitCode === null && !target.killed) {
+        try {
+          target.kill(signal)
+        } catch {
+          // already gone
+        }
+      }
+    })
+  }
+}