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/src/preload/index.js

@@ -95,6 +95,24 @@ contextBridge.exposeInMainWorld('electronAPI', {
     ipcRenderer.on('uninstall-error', (event, data) => callback(data));
   },
   
+
+  // --- VM Disk Sandbox ---
+  vmGetPresets: () => ipcRenderer.invoke('vm:get-presets'),
+  vmGetStatus: () => ipcRenderer.invoke('vm:get-status'),
+  vmProvision: (opts) => ipcRenderer.invoke('vm:provision', opts),
+  vmDestroy: (opts) => ipcRenderer.invoke('vm:destroy', opts),
+  vmExportData: (opts) => ipcRenderer.invoke('vm:export-data', opts),
+  getDiskSpace: (path) => ipcRenderer.invoke('get-disk-space', path),
+  onVMProgress: (callback) => {
+    ipcRenderer.on('vm:progress', (event, data) => callback(data));
+  },
+  onVMComplete: (callback) => {
+    ipcRenderer.on('vm:complete', (event, data) => callback(data));
+  },
+  onVMError: (callback) => {
+    ipcRenderer.on('vm:error', (event, data) => callback(data));
+  },
+
   // Remove listeners
   removeListener: (channel) => {
     ipcRenderer.removeAllListeners(channel);