Fix the MQTT node, suggest message names, and drop Items

- MQTT nodes failed to build: the topic map was renamed to talk in ports, but
  __slots__ still declared the old name, so every MQTT node raised
  AttributeError. Building one of each node type is now a test, since __slots__
  makes this failure invisible until someone places the node.
- Port names offer the messages already in play: everything published is worth
  reading, and an input nobody provides yet is worth publishing. A message only
  connects when both ends spell it the same way, so choosing beats typing.
- Adding a port focuses its name field.
- Dragging onto an input that already reads something offers the extra port as
  well as the replacement — an MQTT or InfluxDB node usually wants both.
- The template's Item model, its routes, screens and table are gone.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016WzrvW7rjQbynnhF6pxh6i

frontend/src/routes/_layout/items.tsx

@@ -1,69 +0,0 @@
-import { useSuspenseQuery } from "@tanstack/react-query"
-import { createFileRoute } from "@tanstack/react-router"
-import { Search } from "lucide-react"
-import { Suspense } from "react"
-
-import { ItemsService } from "@/client"
-import { DataTable } from "@/components/Common/DataTable"
-import AddItem from "@/components/Items/AddItem"
-import { columns } from "@/components/Items/columns"
-import PendingItems from "@/components/Pending/PendingItems"
-
-function getItemsQueryOptions() {
-  return {
-    queryFn: () => ItemsService.readItems({ skip: 0, limit: 100 }),
-    queryKey: ["items"],
-  }
-}
-
-export const Route = createFileRoute("/_layout/items")({
-  component: Items,
-  head: () => ({
-    meta: [
-      {
-        title: "Items - Fluksio",
-      },
-    ],
-  }),
-})
-
-function ItemsTableContent() {
-  const { data: items } = useSuspenseQuery(getItemsQueryOptions())
-
-  if (items.data.length === 0) {
-    return (
-      <div className="flex flex-col items-center justify-center text-center py-12">
-        <div className="rounded-full bg-muted p-4 mb-4">
-          <Search className="h-8 w-8 text-muted-foreground" />
-        </div>
-        <h3 className="text-lg font-semibold">You don't have any items yet</h3>
-        <p className="text-muted-foreground">Add a new item to get started</p>
-      </div>
-    )
-  }
-
-  return <DataTable columns={columns} data={items.data} />
-}
-
-function ItemsTable() {
-  return (
-    <Suspense fallback={<PendingItems />}>
-      <ItemsTableContent />
-    </Suspense>
-  )
-}
-
-function Items() {
-  return (
-    <div className="flex flex-col gap-6">
-      <div className="flex items-center justify-between">
-        <div>
-          <h1 className="text-2xl font-bold tracking-tight">Items</h1>
-          <p className="text-muted-foreground">Create and manage your items</p>
-        </div>
-        <AddItem />
-      </div>
-      <ItemsTable />
-    </div>
-  )
-}