feat: convert file manager into a plugin

src/ui/tests/sidebar/rail-items.test.ts

@@ -48,6 +48,7 @@ describe("RAIL_ITEMS", () => {
     // Guards against a destination silently disappearing now that the rail,
     // the settings toggles, the sidebar titles and the mobile bar all read
     // from this one list.
+    // "sftp" moved to the file-manager plugin's own registerRailItem.
     expect(RAIL_ITEMS.map((item) => item.id)).toEqual([
       "hosts",
       "credentials",
@@ -58,7 +59,6 @@ describe("RAIL_ITEMS", () => {
       "quick-connect",
       "serial",
       "ssh-tools",
-      "sftp",
       "macros",
       "history",
       "session-logs",