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/useVoiceAvailable.ts

@@ -0,0 +1,38 @@
+import { useEffect, useState } from 'react';
+import { authenticatedFetch } from '../../../utils/api';
+
+// Whether the optional voice feature is configured on the server (VOICE_SIDECAR_URL set).
+// Probed once and cached app-wide so the mic/speak controls can hide themselves when off.
+let cached: boolean | null = null;
+let inflight: Promise<boolean> | null = null;
+
+function probe(): Promise<boolean> {
+  if (cached !== null) return Promise.resolve(cached);
+  if (!inflight) {
+    inflight = authenticatedFetch('/api/voice/health')
+      .then((r) => (r.ok ? r.json() : { enabled: false }))
+      .then((d) => {
+        cached = Boolean(d?.enabled);
+        return cached;
+      })
+      .catch(() => {
+        cached = false;
+        return false;
+      });
+  }
+  return inflight;
+}
+
+export function useVoiceAvailable(): boolean {
+  const [available, setAvailable] = useState<boolean>(cached ?? false);
+  useEffect(() => {
+    let mounted = true;
+    probe().then((v) => {
+      if (mounted) setAvailable(v);
+    });
+    return () => {
+      mounted = false;
+    };
+  }, []);
+  return available;
+}