feat(dictation): transcribe after recording instead of live

Parakeet is an offline model trained on whole utterances, so re-decoding
the growing buffer to animate a live transcript cost O(n^2) work for a
result the final decode replaced. Sessions now decode once per committed
segment, and the composer shows a scrolling waveform of the mic level
instead of running text.

Long dictations split at a pause once past 60s (hard cap 90s) instead of
on a blind 15s timer, so cuts no longer land mid-word. Committed segments
decode while the user is still speaking: a 185s dictation returns 4.1s
after stop instead of 11.0s, with identical text (816 vs 817 words).

Also fixes two ways the stream manager could silently drop transcribed
audio. It now counts the commits it issued instead of trusting the
session's echoed events, so a commit still in flight when the client
finishes can no longer be left out of the final text. And segment
byte/peak accounting is reset where the commit is issued rather than when
the event arrives, which could mistake the tail of a dictation for
silence and clear it.

packages/ui/src/lib/dictation/use-dictation-audio-source.ts

@@ -4,20 +4,27 @@
  * Captures mono audio via getUserMedia, taps it with a ScriptProcessorNode
  * (universally supported, including iOS WKWebView), resamples Float32 to
  * 16 kHz PCM16LE, and emits ~1-second base64 chunks plus a normalized RMS
- * volume for the level meter.
+ * level for the waveform.
+ *
+ * The level is delivered by subscription rather than React state: it updates
+ * on every audio callback (~12 Hz), and routing that through state re-rendered
+ * the whole dictation overlay at the same rate.
  */
 
-import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import { useCallback, useEffect, useMemo, useRef } from 'react';
 
 export interface DictationAudioSourceConfig {
     onPcmSegment: (base64Pcm: string) => void;
     onError?: (error: Error) => void;
 }
 
+export type DictationLevelListener = (level: number) => void;
+
 export interface DictationAudioSource {
     start: () => Promise<void>;
     stop: () => Promise<void>;
-    volume: number;
+    /** Subscribe to the normalized (0..1) mic level. Returns an unsubscribe. */
+    subscribeLevel: (listener: DictationLevelListener) => () => void;
 }
 
 const OUTPUT_RATE = 16000;
@@ -125,7 +132,18 @@ export const isDictationCaptureSupported = (): boolean => {
 };
 
 export function useDictationAudioSource(config: DictationAudioSourceConfig): DictationAudioSource {
-    const [volume, setVolume] = useState(0);
+    const levelListenersRef = useRef(new Set<DictationLevelListener>());
+    const emitLevel = useCallback((level: number) => {
+        for (const listener of levelListenersRef.current) {
+            listener(level);
+        }
+    }, []);
+    const subscribeLevel = useCallback((listener: DictationLevelListener) => {
+        levelListenersRef.current.add(listener);
+        return () => {
+            levelListenersRef.current.delete(listener);
+        };
+    }, []);
 
     const onPcmSegmentRef = useRef(config.onPcmSegment);
     const onErrorRef = useRef(config.onError);
@@ -196,7 +214,7 @@ export function useDictationAudioSource(config: DictationAudioSourceConfig): Dic
                     sumSquares += input[i] * input[i];
                 }
                 const rms = Math.sqrt(sumSquares / Math.max(1, input.length));
-                setVolume(Math.min(1, Math.max(0, rms * 2)));
+                emitLevel(Math.min(1, Math.max(0, rms * 2)));
 
                 const next = resampleToPcm16(input, context.sampleRate, OUTPUT_RATE);
                 graph.pending = concatInt16(graph.pending, next);
@@ -227,12 +245,12 @@ export function useDictationAudioSource(config: DictationAudioSourceConfig): Dic
             graphRef.current = emptyGraph();
             throw error instanceof Error ? error : new Error(String(error));
         }
-    }, []);
+    }, [emitLevel]);
 
     const stop = useCallback(async () => {
         const graph = graphRef.current;
         graph.started = false;
-        setVolume(0);
+        emitLevel(0);
 
         if (graph.processor) {
             try {
@@ -272,7 +290,7 @@ export function useDictationAudioSource(config: DictationAudioSourceConfig): Dic
         if (graphRef.current === graph) {
             graphRef.current = emptyGraph();
         }
-    }, []);
+    }, [emitLevel]);
 
     useEffect(() => {
         return () => {
@@ -294,8 +312,8 @@ export function useDictationAudioSource(config: DictationAudioSourceConfig): Dic
                 }
             },
             stop,
-            volume,
+            subscribeLevel,
         }),
-        [start, stop, volume],
+        [start, stop, subscribeLevel],
     );
 }