fix(desktop): harden external editor and session cookie IPC (#1627)

* fix(desktop): stop the renderer choosing the external editor binary or reading the session cookie

open-external-editor now launches only the editor saved from a file dialog
and refuses to hand executable file types to the OS opener. The unused
get-session-cookie and wait-session-cookie channels, which returned the
httpOnly jwt, are removed.

* fix: validate setting keys before editor authorization

electron/external-editor.cjs

@@ -0,0 +1,70 @@
+const path = require("path");
+
+// Saved by the file manager after the user picks an editor in a file dialog.
+const EXTERNAL_EDITOR_SETTING = "fileManager.externalEditorPath";
+
+// Extensions the OS would run rather than open in an editor. With no editor
+// configured the temp copy goes to shell.openPath, and its name comes from
+// the remote server.
+const LAUNCHABLE_EXTENSIONS = new Set([
+  // Windows
+  "application",
+  "appref-ms",
+  "bat",
+  "chm",
+  "cmd",
+  "com",
+  "cpl",
+  "exe",
+  "gadget",
+  "hta",
+  "inf",
+  "jar",
+  "js",
+  "jse",
+  "library-ms",
+  "lnk",
+  "msc",
+  "msi",
+  "msp",
+  "mst",
+  "pif",
+  "ps1",
+  "psd1",
+  "psm1",
+  "py",
+  "pyw",
+  "reg",
+  "scf",
+  "scr",
+  "search-ms",
+  "settingcontent-ms",
+  "url",
+  "vbe",
+  "vbs",
+  "ws",
+  "wsf",
+  "wsh",
+  "xll",
+  // macOS
+  "app",
+  "command",
+  "pkg",
+  "terminal",
+  "tool",
+  "workflow",
+  // Linux
+  "appimage",
+  "desktop",
+  "run",
+  "sh",
+]);
+
+function isLaunchableFileName(fileName) {
+  // Windows drops trailing dots and spaces, so "a.exe. " is saved as a.exe.
+  const name = path.basename(String(fileName || "")).replace(/[. ]+$/, "");
+  const extension = path.extname(name).slice(1).toLowerCase();
+  return LAUNCHABLE_EXTENSIONS.has(extension);
+}
+
+module.exports = { EXTERNAL_EDITOR_SETTING, isLaunchableFileName };