Flow inputs and outputs are visible and editable in the UI

A flow's inputs are the messages it takes from outside — a dashboard control,
a run, the API — and its outputs are what a batch run reports. Both existed in
the document and in the engine, and neither had any UI: the values looked
hard-coded on the canvas and the Run button always used the declared defaults.

The canvas now draws each as a labelled endpoint, the way it already draws a
dashboard tile or another flow, skipping an input something else already
accounts for. The flow panel edits them — mode, name, type, starting value,
and for a live flow the value it currently holds with a way to put a new one
in. Pressing Run on a batch flow asks for its parameters first.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NUb8YpL2s3gmN9WTACTt4q

frontend/tests/endpoints.spec.ts

@@ -29,12 +29,19 @@ test.beforeAll(async ({ browser }) => {
         {
           id: "sink",
           type: "python",
-          requires: [{ name: "level", port: "level", dtype: "float" }],
+          requires: [
+            { name: "level", port: "level", dtype: "float" },
+            { name: "ceiling", port: "ceiling", dtype: "float" },
+          ],
           provides: [],
         },
       ],
       inputs: [
         { spec: { name: "level", port: "level", dtype: "float" }, initial: 0 },
+        {
+          spec: { name: "ceiling", port: "ceiling", dtype: "float" },
+          initial: 30,
+        },
       ],
       version: 1,
     },
@@ -88,6 +95,19 @@ test("a dashboard control is drawn on the flow it feeds", async ({ page }) => {
   await expect(page.getByText("Lever")).toBeVisible()
 })
 
+test("an input nothing else writes is drawn, and the wired one is not", async ({
+  page,
+}) => {
+  await page.goto(`/flows/${flowName}`)
+  await page.waitForSelector(".react-flow__node")
+
+  // The flow declares it and no node computes it, so it needs a visible
+  // origin. `level` already has one — the control below — and a second label
+  // saying the same thing would only be in the way.
+  await expect(page.getByText("ceiling")).toBeVisible()
+  await expect(page.getByText("level")).toHaveCount(0)
+})
+
 test("the dashboard is never stored as a node of the flow", async ({
   page,
 }) => {