feat(voice): add optional speech-to-text input and read-aloud TTS

Adds a push-to-talk mic button in the composer and a read-aloud button on
assistant messages. Both are opt-in and hidden unless a voice backend is
configured via VOICE_SIDECAR_URL.

The auth-gated /api/voice proxy forwards to a configurable backend exposing
/transcribe and /tts (provider-agnostic); the frontend probes /api/voice/health
and hides the controls when disabled. Adds i18n keys and docs/voice.md.

Includes a local, no-API-key reference backend in voice-sidecar/ (faster-whisper
for STT, Kokoro-82M for TTS, both CPU-capable).

src/components/chat/hooks/useVoiceInput.ts

@@ -0,0 +1,106 @@
+import { useCallback, useRef, useState } from 'react';
+import { authenticatedFetch } from '../../../utils/api';
+
+// Mobile-safe recording: iOS Safari 18.4+ supports webm/opus; older iOS needs mp4.
+const MIME_CANDIDATES = [
+  'audio/webm;codecs=opus',
+  'audio/webm',
+  'audio/mp4',
+  'audio/ogg;codecs=opus',
+  'audio/ogg',
+];
+
+function pickMime(): string {
+  for (const t of MIME_CANDIDATES) {
+    try {
+      if (typeof MediaRecorder !== 'undefined' && MediaRecorder.isTypeSupported(t)) return t;
+    } catch {
+      /* isTypeSupported can throw on some iOS versions */
+    }
+  }
+  return '';
+}
+
+export type VoiceInputState = 'idle' | 'recording' | 'transcribing';
+
+/**
+ * Push-to-talk dictation. Records the mic, uploads to /api/voice/transcribe
+ * (faster-whisper sidecar via the Express proxy), returns text via onTranscript.
+ * Ported from tooler's VoiceInput.js.
+ */
+export function useVoiceInput(onTranscript: (text: string) => void, onError?: (msg: string) => void) {
+  const [state, setState] = useState<VoiceInputState>('idle');
+  const recorderRef = useRef<MediaRecorder | null>(null);
+  const chunksRef = useRef<Blob[]>([]);
+  const streamRef = useRef<MediaStream | null>(null);
+
+  const stopTracks = () => {
+    streamRef.current?.getTracks().forEach((t) => t.stop());
+    streamRef.current = null;
+  };
+
+  const start = useCallback(async () => {
+    try {
+      const stream = await navigator.mediaDevices.getUserMedia({
+        audio: { echoCancellation: true, noiseSuppression: true },
+      });
+      streamRef.current = stream;
+      const mimeType = pickMime();
+      const rec = mimeType ? new MediaRecorder(stream, { mimeType }) : new MediaRecorder(stream);
+      recorderRef.current = rec;
+      chunksRef.current = [];
+
+      rec.ondataavailable = (e) => {
+        if (e.data.size > 0) chunksRef.current.push(e.data);
+      };
+
+      rec.onstop = async () => {
+        stopTracks();
+        const type = rec.mimeType || 'audio/webm';
+        const blob = new Blob(chunksRef.current, { type });
+        if (blob.size < 800) {
+          setState('idle');
+          onError?.('Recording too short');
+          return;
+        }
+        setState('transcribing');
+        try {
+          const ext = type.includes('mp4') ? 'm4a' : type.includes('ogg') ? 'ogg' : 'webm';
+          const fd = new FormData();
+          fd.append('audio', blob, `recording.${ext}`);
+          const res = await authenticatedFetch('/api/voice/transcribe', { method: 'POST', body: fd });
+          if (!res.ok) throw new Error(`transcribe ${res.status}`);
+          const data = await res.json();
+          const text = String(data?.text || '').trim();
+          if (text) onTranscript(text);
+          else onError?.('No speech detected');
+        } catch (e) {
+          onError?.(`Transcription failed: ${e instanceof Error ? e.message : String(e)}`);
+        } finally {
+          setState('idle');
+        }
+      };
+
+      rec.start();
+      setState('recording');
+    } catch (e) {
+      const err = e as { name?: string; message?: string };
+      let msg = `Mic error: ${err?.message || e}`;
+      if (err?.name === 'NotAllowedError') msg = 'Microphone access denied.';
+      else if (err?.name === 'NotFoundError') msg = 'No microphone found.';
+      onError?.(msg);
+      setState('idle');
+    }
+  }, [onTranscript, onError]);
+
+  const stop = useCallback(() => {
+    if (recorderRef.current && state === 'recording') recorderRef.current.stop();
+  }, [state]);
+
+  const toggle = useCallback(() => {
+    if (state === 'recording') stop();
+    else if (state === 'idle') start();
+  }, [state, start, stop]);
+
+  return { state, toggle };
+}