Open saved session: visible progress + clear errors (no silent no-op)

The status line only rendered inside the last-in-memory-session block, so 'Open
saved session' processed invisibly — looked like nothing happened. Now: the
transcript status (with a spinner) is always shown, the processing(0,0) reconcile
phase reads 'Working… (this can take a few minutes)', and invalid picks surface an
alert (not a recorded session / already processing / unreadable transcript) instead
of doing nothing.

Ten31Transcripts/Session/SessionController.swift

@@ -444,13 +444,16 @@ final class SessionController: ObservableObject {
         if let folder = lastSession?.folder { openEditor(folder: folder) }
     }
 
-    /// Open the editor for any session folder that has a `speakers.json`.
-    private func openEditor(folder: URL) {
+    /// Open the editor for any session folder that has a `speakers.json`. Returns
+    /// false if it couldn't load (no/empty transcript).
+    @discardableResult
+    private func openEditor(folder: URL) -> Bool {
         guard let model = RecapEditModel(folder: folder, voiceprints: voiceprints,
                                          baseURL: settings.backendBaseURL, skipTLS: settings.skipTLSVerification,
                                          templates: settings.recapTemplates, defaultTemplateId: settings.defaultTemplateId)
-        else { return }
+        else { return false }
         EditorWindow.shared.show(model: model)
+        return true
     }
 
     /// Pick any past session folder and open it: edit it if already transcribed,
@@ -467,13 +470,22 @@ final class SessionController: ObservableObject {
         guard panel.runModal() == .OK, let folder = panel.url else { return }
         let fm = FileManager.default
         if fm.fileExists(atPath: folder.appendingPathComponent("speakers.json").path) {
-            openEditor(folder: folder)
+            if !openEditor(folder: folder) {
+                Self.alert("Couldn't open this session — its transcript looks empty or unreadable.")
+            }
             return
         }
         // Not transcribed yet — needs the raw tracks to (re)process.
         let mic = folder.appendingPathComponent("mic.wav")
         let sys = folder.appendingPathComponent("system.wav")
-        guard fm.fileExists(atPath: mic.path), fm.fileExists(atPath: sys.path), !isProcessing else { return }
+        guard fm.fileExists(atPath: mic.path), fm.fileExists(atPath: sys.path) else {
+            Self.alert("“\(folder.lastPathComponent)” isn’t a recorded session (no mic.wav / system.wav).")
+            return
+        }
+        guard !isProcessing else {
+            Self.alert("Already processing a session — let it finish first.")
+            return
+        }
         transcriptStatus = .processing(0, 1)
         recapURL = nil
         let selfName = settings.selfName
@@ -500,6 +512,15 @@ final class SessionController: ObservableObject {
         folder.lastPathComponent.split(separator: "_").last.map(String.init) ?? "manual"
     }
 
+    private static func alert(_ message: String) {
+        let a = NSAlert()
+        a.messageText = "Open saved session"
+        a.informativeText = message
+        a.alertStyle = .informational
+        NSApp.activate(ignoringOtherApps: true)
+        a.runModal()
+    }
+
     private func fail(_ message: String) {
         recorder = nil
         visualCapture = nil   // recorder.start() failed before visual started; nothing running