feat: add integrated terminal backend

- add a PTY manager with platform shell resolution and streamed data/exit events
- expose terminal lifecycle and terminal height IPC through preload and app service
- persist terminal panel height in workspace settings and document the backend contract
- add backend tests and validate native packaging with bun run package

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -66,6 +66,7 @@
     "yaml": "^2.8.3"
   },
   "dependencies": {
-    "keytar": "^7.9.0"
+    "keytar": "^7.9.0",
+    "node-pty": "^1.1.0"
   }
 }