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/components/Sidebar/AppSidebar.tsx

@@ -1,4 +1,4 @@
-import { Briefcase, Home, Users, Workflow } from "lucide-react"
+import { Home, Users, Workflow } from "lucide-react"
 
 import { SidebarAppearance } from "@/components/Common/Appearance"
 import { Logo } from "@/components/Common/Logo"
@@ -15,7 +15,6 @@ import { User } from "./User"
 const baseItems: Item[] = [
   { icon: Home, title: "Dashboard", path: "/" },
   { icon: Workflow, title: "Flows", path: "/flows" },
-  { icon: Briefcase, title: "Items", path: "/items" },
 ]
 
 export function AppSidebar() {