Make diarization chunk length configurable (Auto + presets)

Chunk size was hardcoded at 2.5-min bodies. Add a Settings control:
Auto / Standard 2.5min / Large group 60s / Fine 90s. Shorter chunks keep fewer
simultaneous speakers per window (Sortformer resolves ~4/chunk), useful for large
calls, at some cost to speed and cross-chunk voice matching.

- ChunkMode (new, pure/testable): mode → body seconds; Auto picks 60s when >4
  participants were detected, else 150s; overlap + single-chunk threshold scale
  with the body length.
- AppSettings.chunkMode (+ typed `chunk`); SettingsView picker with explanation.
- TranscriptPipeline.process gains chunkSeconds; derives overlap/threshold from it.
- SessionController resolves the body from the setting + the session's detected
  participant count (visual_timeline participants) for both send + re-process.
- Participant roster now counts EVERY tile OCR'd, not just who spoke
  (TimelineBuilder.observedNames → VisualObserver → VisualCapture), so the Auto
  call-size signal is meaningful even though speaking-detection is sparse.

Tests: ChunkMode resolution, overlap scaling, short-body re-chunking. 69 pass.

Ten31Transcripts/Session/TranscriptPipeline.swift

@@ -28,6 +28,7 @@ final class TranscriptPipeline {
                  selfSpans: [VADSpan],
                  selfName: String,
                  systemHealthy: Bool,
+                 chunkSeconds: Double = 150,
                  progress: ((Int, Int) async -> Void)? = nil) async throws -> SpeakersFile {
         let fm = FileManager.default
         let dual = systemHealthy
@@ -36,7 +37,12 @@ final class TranscriptPipeline {
         let duration = dual
             ? max(SessionPackager.duration(of: micURL), SessionPackager.duration(of: systemURL))
             : SessionPackager.duration(of: mixedURL)
-        let plan = SessionPackager.planChunks(durationSec: duration)
+        // Chunk to the requested body length; overlap and the single-chunk threshold
+        // scale with it (a 60s body shouldn't be cut by a fixed 15s margin or stay
+        // unchunked below the 2.5-min default threshold).
+        let overlap = ChunkMode.overlapSeconds(forBody: chunkSeconds)
+        let plan = SessionPackager.planChunks(durationSec: duration, chunkSeconds: chunkSeconds,
+                                              overlapSeconds: overlap, thresholdSec: chunkSeconds * 1.2)
 
         // Zero-duration / empty session → a valid empty speakers.json, no backend call.
         if plan.isEmpty || duration <= 0 {