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/Settings/AppSettings.swift

@@ -60,6 +60,15 @@ final class AppSettings: ObservableObject {
         didSet { defaults.set(reconcileSpeakers, forKey: Keys.reconcileSpeakers) }
     }
 
+    /// Diarization chunk length (raw value of `ChunkMode`). `.auto` shrinks chunks on
+    /// large calls so a window is less likely to exceed Sortformer's ~4-speaker cap.
+    @Published var chunkMode: String {
+        didSet { defaults.set(chunkMode, forKey: Keys.chunkMode) }
+    }
+
+    /// Typed accessor for `chunkMode`.
+    var chunk: ChunkMode { ChunkMode(rawValue: chunkMode) ?? .auto }
+
     /// User-editable recap templates (takeaways categories per meeting type).
     @Published var recapTemplates: [RecapTemplate] {
         didSet { persist(recapTemplates, forKey: Keys.recapTemplates) }
@@ -104,6 +113,7 @@ final class AppSettings: ObservableObject {
         self.autoSendOnStop = defaults.object(forKey: Keys.autoSend) as? Bool ?? false
         self.recapEnabled = defaults.object(forKey: Keys.recapEnabled) as? Bool ?? true
         self.reconcileSpeakers = defaults.object(forKey: Keys.reconcileSpeakers) as? Bool ?? true
+        self.chunkMode = defaults.string(forKey: Keys.chunkMode) ?? ChunkMode.auto.rawValue
 
         let loaded = (defaults.data(forKey: Keys.recapTemplates))
             .flatMap { try? JSONDecoder().decode([RecapTemplate].self, from: $0) }
@@ -126,6 +136,7 @@ final class AppSettings: ObservableObject {
         static let autoSend = "autoSendOnStop"
         static let recapEnabled = "recapEnabled"
         static let reconcileSpeakers = "reconcileSpeakers"
+        static let chunkMode = "chunkMode"
         static let recapTemplates = "recapTemplates"
         static let defaultTemplate = "defaultTemplateId"
     }