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/scripts/_check.mjs

@@ -0,0 +1,29 @@
+import { chromium } from "@playwright/test"
+const APP = "http://app.localhost"
+const OUT = "/tmp/claude-1000/-home-lc3267-Documents-CodeWorkspace-fluksio/8b6d42d6-2889-4321-ac59-4e6692fcfe3c/scratchpad"
+const browser = await chromium.launch()
+const ctx = await browser.newContext({ viewport: { width: 1440, height: 900 } })
+const page = await ctx.newPage()
+page.on("pageerror", (e) => console.log(`[pageerror] ${e.message}`))
+await page.goto(`${APP}/login`)
+await page.getByTestId("email-input").fill(process.env.FIRST_SUPERUSER)
+await page.getByTestId("password-input").fill(process.env.FIRST_SUPERUSER_PASSWORD)
+await page.getByRole("button", { name: /log in/i }).click()
+await page.waitForURL(`${APP}/`)
+
+console.log("sidebar items entry:", await page.getByRole("link", { name: "Items" }).count())
+
+await page.goto(`${APP}/flows/heating`)
+await page.waitForSelector(".react-flow__node")
+await page.waitForTimeout(1200)
+
+// A node that consumes: its Consumes list should suggest what others publish.
+await page.locator(".react-flow__node").filter({ hasText: "Notify" }).first().click()
+const panel = page.getByTestId("node-panel")
+await panel.getByRole("button", { name: "Add" }).first().click()
+await page.waitForTimeout(600)
+console.log("focused after Add:", await page.evaluate(() => document.activeElement?.getAttribute("aria-label")))
+const options = await page.getByRole("option").allInnerTexts()
+console.log("suggestions offered:", options)
+await page.screenshot({ path: `${OUT}/suggestions.png` })
+await browser.close()