feat: move fleets into a first-party plugin

plugins/fleets/frontend/index.mjs

@@ -0,0 +1,40 @@
+/**
+ * Frontend half of the fleets plugin.
+ *
+ * FleetsPanel is a left-sidebar railView panel (opened from the rail, not a
+ * Tab), the same shape as HostsPanel/CredentialsPanel/AiPanel. None of those
+ * go through registerRailItem/registerTabComponent either -- per
+ * src/ui/shell/pluginLoader.ts's own comment, a rail-view panel like this is
+ * rendered directly by AppShell.tsx rather than registered at runtime, the
+ * same way ai's AiPanel is. FleetsPanel is reached today through a direct
+ * relative import in AppShell.tsx (../../plugins/fleets/frontend/FleetsPanel),
+ * not through register() below.
+ *
+ * register()/unregister() still follow the same shape every other plugin's
+ * frontend entry does, so this is ready the moment a real frontend plugin
+ * loader exists -- 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".
+ *
+ * The fleet-inventory tab (opened from within FleetsPanel) is wired
+ * separately in src/ui/shell/tabUtils.tsx, the same way docker/host-metrics
+ * tabs are -- a plugin tab id that already exists in the built-in TabType
+ * union goes through the ordinary tabSurfaceLoaders/switch wiring there, not
+ * through registerTabComponent (which is for tab ids outside that union).
+ */
+
+export const id = "fleets";
+export const railViewId = "fleets";
+
+export async function register({ registerRailItem, icons }) {
+  registerRailItem?.({
+    id: railViewId,
+    icon: icons.Boxes,
+    labelKey: "nav.fleets",
+    separatorAfter: true,
+  });
+}
+
+export async function unregister({ unregisterRailItem }) {
+  unregisterRailItem?.(railViewId);
+}