feat: add start/stop/restart buttons for VMs and LXCs

- New POST /api/power route for Proxmox power operations
- powerControl() method in proxmox.js library
- Start/Stop/Restart action buttons in VMList and LXCList tables
- Status feedback banner in Dashboard on power action

Co-Authored-By: Claude <noreply@anthropic.com>

src/lib/proxmox.js

@@ -163,7 +163,20 @@ export async function getLXC(nodeName) {
  * Aggregated summary across all nodes.
  * @returns {Promise<SummaryStat>}
  */
-export async function getSummary() {
+/**
+ * Sends a power action to a VM or LXC.
+ * @param {string} nodeName
+ * @param {number} vmid
+ * @param {'qemu'|'lxc'} type
+ * @param {'start'|'stop'|'restart'} action
+ * @returns {Promise<void>}
+ */
+export async function powerControl(nodeName, vmid, type, action) {
+  await proxmoxFetch(`/nodes/${nodeName}/${type}/${vmid}/status/${action}`, 'POST');
+}
+
+/**
+ * Aggregated summary across all nodes.
   const nodes = await getNodes();
   let totalVMs = [];
   let totalLXC = [];