fix(desktop): prevent backend zombie process and IPC listener leaks

desktop/src/main/preload.ts

@@ -7,12 +7,18 @@ contextBridge.exposeInMainWorld('electronAPI', {
   selectDirectory: () => ipcRenderer.invoke('select-directory'),
   selectFile: (filters?: Electron.FileFilter[]) => ipcRenderer.invoke('select-file', filters),
 
+  // Each listener registrar returns an unsubscribe fn so renderers can clean
+  // up on unmount / effect re-run and avoid accumulating duplicate handlers.
   onBackendStatus: (callback: (data: { status: string; port?: number; error?: string }) => void) => {
-    ipcRenderer.on('backend-status', (_event, data) => callback(data))
+    const handler = (_event: unknown, data: { status: string; port?: number; error?: string }) => callback(data)
+    ipcRenderer.on('backend-status', handler)
+    return () => ipcRenderer.removeListener('backend-status', handler)
   },
 
   onBackendLog: (callback: (line: string) => void) => {
-    ipcRenderer.on('backend-log', (_event, line) => callback(line))
+    const handler = (_event: unknown, line: string) => callback(line)
+    ipcRenderer.on('backend-log', handler)
+    return () => ipcRenderer.removeListener('backend-log', handler)
   },
 
   // Window controls (custom titlebar on Windows)
@@ -21,12 +27,16 @@ contextBridge.exposeInMainWorld('electronAPI', {
   windowClose: () => ipcRenderer.invoke('window-close'),
   windowIsMaximized: () => ipcRenderer.invoke('window-is-maximized'),
   onMaximizeChange: (callback: (maximized: boolean) => void) => {
-    ipcRenderer.on('window-maximize-changed', (_event, max) => callback(max))
+    const handler = (_event: unknown, max: boolean) => callback(max)
+    ipcRenderer.on('window-maximize-changed', handler)
+    return () => ipcRenderer.removeListener('window-maximize-changed', handler)
   },
 
   // App menu / shortcut actions forwarded from the main process.
   onMenuAction: (callback: (action: string) => void) => {
-    ipcRenderer.on('menu-action', (_event, action: string) => callback(action))
+    const handler = (_event: unknown, action: string) => callback(action)
+    ipcRenderer.on('menu-action', handler)
+    return () => ipcRenderer.removeListener('menu-action', handler)
   },
 
   platform: process.platform,

desktop/src/main/python-manager.ts

@@ -176,11 +176,15 @@ export class PythonBackend extends EventEmitter {
   }
 
   stop(): void {
-    if (this.process) {
-      this.process.kill('SIGTERM')
+    const proc = this.process
+    if (proc) {
+      proc.kill('SIGTERM')
+      // Keep a local ref so the SIGKILL fallback can still reach the process
+      // even after we clear `this.process`; otherwise a stuck backend would
+      // never be force-killed and leak as a zombie.
       setTimeout(() => {
-        if (this.process && !this.process.killed) {
-          this.process.kill('SIGKILL')
+        if (!proc.killed) {
+          proc.kill('SIGKILL')
         }
       }, 5000)
       this.process = null