feat(desktop): add auto-speak watcher hook

apps/desktop/src/app/chat/composer/hooks/use-auto-speak-replies.ts

@@ -0,0 +1,79 @@
+import { useStore } from '@nanostores/react'
+import { useEffect, useRef } from 'react'
+
+import { playSpeechText } from '@/lib/voice-playback'
+import { notifyError } from '@/store/notifications'
+import { $messages } from '@/store/session'
+import { $voicePlayback } from '@/store/voice-playback'
+import { $autoSpeakReplies } from '@/store/voice-prefs'
+
+interface AutoSpeakReply {
+  id: string
+  pending: boolean
+  text: string
+}
+
+interface UseAutoSpeakReplies {
+  conversationActive: boolean
+  failureLabel: string
+  /** Mark the current last reply spoken — shared dedupe with the conversation consumer. */
+  markSpoken: () => void
+  /** Latest completed assistant reply, or null; `pending` true while still streaming. */
+  pendingReply: () => AutoSpeakReply | null
+  /** Re-arm on session switch so opening a chat never reads its existing last reply. */
+  sessionId: string | null | undefined
+}
+
+/**
+ * Pure-TTS auto-speak: when `voice.auto_tts` is on, read each completed assistant
+ * turn aloud — no dictation, no conversation loop. Stays off while a full voice
+ * conversation runs (it speaks replies itself) and never overlaps clips: a reply
+ * landing mid-playback is held and spoken on the playback-idle edge. Always reads
+ * the latest reply, so a backlog collapses to the newest.
+ */
+export function useAutoSpeakReplies({
+  conversationActive,
+  failureLabel,
+  markSpoken,
+  pendingReply,
+  sessionId
+}: UseAutoSpeakReplies) {
+  const enabled = useStore($autoSpeakReplies)
+  const latest = useRef({ conversationActive, failureLabel, markSpoken, pendingReply })
+  latest.current = { conversationActive, failureLabel, markSpoken, pendingReply }
+
+  useEffect(() => {
+    if (!enabled) {
+      return undefined
+    }
+
+    // Don't read whatever reply already sits at the bottom when the toggle flips
+    // on (or a chat opens) — consume it so only later replies are spoken.
+    latest.current.markSpoken()
+
+    const speakLatest = () => {
+      const { conversationActive, failureLabel, markSpoken, pendingReply } = latest.current
+
+      if (conversationActive || $voicePlayback.get().status !== 'idle') {
+        return
+      }
+
+      const reply = pendingReply()
+
+      if (!reply || reply.pending) {
+        return
+      }
+
+      markSpoken()
+      void playSpeechText(reply.text, { messageId: reply.id, source: 'read-aloud' }).catch(error =>
+        notifyError(error, failureLabel)
+      )
+    }
+
+    // Re-check on a reply completing ($messages) and on the prior clip ending
+    // ($voicePlayback → idle), which frees us to read the next held reply.
+    const stops = [$messages.subscribe(speakLatest), $voicePlayback.listen(speakLatest)]
+
+    return () => stops.forEach(f => f())
+  }, [enabled, sessionId])
+}