Voice nodes (openpanel V4): stt, tts, wren reply/unhandled, socket publish

- `stt`: one realtime transcription session per utterance on the speech
  server's url, opened at `listening` true or the first chunk, committed at
  false; a whole clip goes to /v1/audio/transcriptions. Returns at once and
  publishes `partial` and `text` when the server answers.
- `tts`: one /v1/audio/speech call per sentence, cached by voice and text in
  node state; a reference is marked `append` while earlier ones still play.
- wren: `reply` says what was applied, `unhandled` carries the prompt when
  nothing was. The install hint names the git pin (PyPI's `wren` is another
  package).
- ai/client: transcribe, speak and transcribe_stream beside stream_completion.
- A device socket publishes scalar `in` ports as {"type": "publish"} text,
  in order with its binary frames.
- Pipeline.publish walks only what its values reach: a node downstream of a
  consumer that published nothing no longer runs again (wren re-acted on
  every microphone chunk).

frontend/src/components/Flow/FlowNode.tsx

@@ -12,6 +12,7 @@ import {
   Filter,
   Globe,
   Merge,
+  Mic,
   MonitorSmartphone,
   Play,
   Plug,
@@ -22,6 +23,7 @@ import {
   Square,
   Terminal,
   Timer,
+  Volume2,
 } from "lucide-react"
 import { memo, useEffect, useRef, useState } from "react"
 
@@ -63,6 +65,8 @@ const NODE_ICONS = {
   file: FileText,
   ntfy: Bell,
   wren: Bird,
+  stt: Mic,
+  tts: Volume2,
   // The glyph the Devices tab goes by.
   device: MonitorSmartphone,
 } as const