feat(subtitles): refine dialogue-focused subtitle generation

- Split long speech at quiet pauses with overlapping context
- Stitch overlapping cues without merging repeated dialogue
- Clarify dialogue controls and hide the sidebar button when cues load

src/core/services/subtitle-generation-pauses.ts

@@ -0,0 +1,35 @@
+import { runSubtitleGenerationProcess } from './subtitle-generation-process';
+
+// Each completed silencedetect line contains both the end and duration of a quiet interval.
+export async function findSpeechPauses(input: {
+  ffmpegPath: string;
+  wavPath: string;
+  signal?: AbortSignal;
+}): Promise<number[]> {
+  const pauses: number[] = [];
+  await runSubtitleGenerationProcess({
+    command: input.ffmpegPath.trim() || 'ffmpeg',
+    args: [
+      '-nostdin',
+      '-hide_banner',
+      '-nostats',
+      '-i',
+      input.wavPath,
+      '-af',
+      'silencedetect=noise=-35dB:d=0.12',
+      '-f',
+      'null',
+      '-',
+    ],
+    signal: input.signal,
+    onLine: (line) => {
+      const match = /silence_end: (\S+) \| silence_duration: (\S+)/.exec(line);
+      if (!match) return;
+      const end = Number(match[1]);
+      const duration = Number(match[2]);
+      if (Number.isFinite(end) && Number.isFinite(duration) && duration > 0 && end >= duration)
+        pauses.push(end - duration / 2);
+    },
+  });
+  return pauses.sort((a, b) => a - b);
+}