feat(voice): improve TTS logging for root-cause diagnosis

- Route now logs every synthesis request (char count, voice, speed)
- Route logs char count + text preview when the 8000-char limit is hit
- Route logs empty audio with preview (helps spot no-chunk-produced edge case)
- Route logs success with byte count and duration
- Kokoro synthesise() logs per-call: samples produced, elapsed, chars/s
- Kokoro synthesise() logs warning when zero audio chunks returned with preview
- Kokoro synthesise() catches and logs pipeline-internal errors with preview
- Frontend: console.warn now includes char count + 80-char preview on failure and retry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/composables/useStreamingTts.ts

@@ -86,11 +86,21 @@ export function useStreamingTts(options: UseStreamingTtsOptions): UseStreamingTt
     try {
       blob = await synthesiseSpeech(stripped)
     } catch (e) {
-      console.warn('[StreamingTTS] Synthesis failed, retrying sentence', { sentence: stripped, error: e })
+      const errMsg = e instanceof Error ? e.message : String(e)
+      console.warn('[StreamingTTS] Synthesis failed, retrying', {
+        chars: stripped.length,
+        preview: stripped.slice(0, 80),
+        error: errMsg,
+      })
       try {
         blob = await synthesiseSpeech(stripped)
       } catch (e2) {
-        console.warn('[StreamingTTS] Retry also failed, skipping sentence', { sentence: stripped, error: e2 })
+        const errMsg2 = e2 instanceof Error ? e2.message : String(e2)
+        console.warn('[StreamingTTS] Retry failed, sentence dropped', {
+          chars: stripped.length,
+          preview: stripped.slice(0, 80),
+          error: errMsg2,
+        })
       }
     } finally {
       pendingCount.value--