Podcast UX baseline: mini player during generation, end-of-track reset, library view

- Mini player shows as soon as generation starts (not just when audio ready)
- Idempotency: tapping headphones again during generation is a no-op
- Playback resets to position 0 on completion (standard player behavior)
- Stop button in full player visible during generation
- PodcastLibraryView lists all generated podcasts
- Sparkles menu gains Podcasts entry
- Fix AnalyticsService Sendable error (serialize before Task.detached boundary)

Marks/Services/AnalyticsService.swift

@@ -2,6 +2,10 @@ import Foundation
 
 enum Analytics {
     static func track(_ event: String, _ properties: [String: Any] = [:]) {
+        // Serialize before crossing the task boundary — Data is Sendable, [String: Any] is not
+        var body: [String: Any] = ["event": event]
+        if !properties.isEmpty { body["properties"] = properties }
+        guard let payload = try? JSONSerialization.data(withJSONObject: body) else { return }
         Task.detached {
             do {
                 let token = try await MarksAuth.validToken()
@@ -10,9 +14,7 @@ enum Analytics {
                 request.httpMethod = "POST"
                 request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
                 request.setValue("application/json", forHTTPHeaderField: "Content-Type")
-                var body: [String: Any] = ["event": event]
-                if !properties.isEmpty { body["properties"] = properties }
-                request.httpBody = try JSONSerialization.data(withJSONObject: body)
+                request.httpBody = payload
                 let (_, response) = try await URLSession.shared.data(for: request)
                 let status = (response as? HTTPURLResponse)?.statusCode ?? 0
                 if status < 200 || status > 299 {