Surface whether visual capture ran on the last session

Visual capture falls back to audio-only silently, so the user couldn't tell if
it attached on a real call. SessionInfo now carries visualSegmentCount (nil =
audio-only; a count = visual ran, with that many vision-detected speaker
segments), shown in the menu as '… · N visual segments' or '… · audio-only'.
Makes the pending live-call validation unambiguous.

Ten31Transcripts/UI/MenuBarView.swift

@@ -83,7 +83,7 @@ struct MenuBarView: View {
                 Button {
                     NSWorkspace.shared.activateFileViewerSelecting([last.mixedURL])
                 } label: {
-                    Text("Last: \(Int(last.duration.rounded()))s · \(last.selfSpanCount) self-spans — reveal in Finder")
+                    Text("Last: \(Int(last.duration.rounded()))s · \(last.selfSpanCount) self-spans · \(last.visualSegmentCount.map { "\($0) visual segments" } ?? "audio-only") — reveal in Finder")
                         .font(.caption)
                 }
                 .buttonStyle(.link)