feat: move proxmox discovery/import/sync into a first-party plugin

plugins/proxmox/frontend/index.mjs

@@ -0,0 +1,44 @@
+/**
+ * Frontend half of the proxmox plugin.
+ *
+ * Unlike ssh-terminal/docker, proxmox has no rail tab or standalone view —
+ * only a discovery dialog (opened imperatively from HostsPanel.tsx, not
+ * through a tab seam) and a host-editor settings tab. Registers the
+ * host-editor tab through the same registerHostEditorTab seam docker uses.
+ *
+ * HostProxmoxTab itself stays in src/ui/sidebar/HostEditorFeatureTabs.tsx
+ * rather than moving into this plugin: it takes the same {form, setField}
+ * pair every other host editor tab does, and those types (HostEditorForm,
+ * the setField signature) are core types HostEditor.tsx owns, the same
+ * reason ssh-terminal leaves session-manager.ts in core rather than copying
+ * it here.
+ *
+ * register() is called when the plugin is enabled, unregister() when it is
+ * disabled. As of this change neither is actually invoked anywhere yet --
+ * see plugins/ssh-terminal/frontend/index.mjs and
+ * src/ui/tests/sidebar/plugin-extension-seam.test.tsx, which says outright
+ * "there is no plugin loader yet" for the frontend half. This file follows
+ * the same shape so it is ready the moment that loader exists. The dialog
+ * is reached today through a direct import in HostsPanel.tsx, not through
+ * this seam.
+ */
+
+export const id = "proxmox";
+export const hostEditorTabId = "proxmox";
+
+export async function register({ registerHostEditorTab, icons }) {
+  if (registerHostEditorTab) {
+    const { HostProxmoxTab } =
+      await import("../../../src/ui/sidebar/HostEditorFeatureTabs.tsx");
+    registerHostEditorTab({
+      id: hostEditorTabId,
+      labelKey: "hosts.tabProxmox",
+      icon: icons.Server,
+      component: HostProxmoxTab,
+    });
+  }
+}
+
+export async function unregister({ unregisterHostEditorTab }) {
+  unregisterHostEditorTab?.(hostEditorTabId);
+}