feat: VM disk sandboxing with full VM isolation

- Add VM provisioning module (vm-provisioner.js) with 3 platform strategies:
  * Windows: WSL2 distro with fixed VHDX
  * macOS: Lima VM with fixed disk
  * Linux: loopback ext4 image
- Add IPC handlers (vm-ipc.js) for Electron wizard integration
- Add disk budget wizard step (disk-budget-step.js) with presets
- Wire VM handlers into main process (index.js)
- Add preload bridges for VM operations
- Update install.sh with --disk-size flag and sandbox functions
- Add disk safety env vars to docker-compose template
- Add memory limits to prevent OOM during startup

Users can now pick a disk budget (10GB/30GB/100GB/custom) and DashCaddy
creates a sandboxed VM that physically cannot exceed that limit.
Uninstall cleanly removes the entire VM/disk with zero leakage.

dashcaddy-installer/.gitignore

@@ -5,3 +5,4 @@ dist/
 Thumbs.db
 LOGO_INTEGRATION.md
 README-TESTER.txt
+build-output