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/src/components/Flow/EndpointNode.tsx

@@ -1,5 +1,5 @@
 import { Handle, type NodeProps, Position } from "@xyflow/react"
-import { LayoutDashboard, Workflow } from "lucide-react"
+import { LayoutDashboard, LogIn, LogOut, Workflow } from "lucide-react"
 import { memo } from "react"
 
 import { useIsMobile } from "@/hooks/useMobile"
@@ -9,6 +9,8 @@ import type { EndpointNodeData } from "./endpoints"
 const KIND_ICONS = {
   dashboard: LayoutDashboard,
   flow: Workflow,
+  input: LogIn,
+  output: LogOut,
 } as const
 
 /**