feat(app,android,ios): say when playback fails and offer to retry from there

A file mpv gave up on partway ended the desktop player without a word:
the page went back to the title and the film was simply gone. It now
leaves an error toast naming where it stopped, with Retry reopening the
episode at that position.

The phone players already showed an error, but its only button was
Dismiss, so a dropped connection meant backing out and finding the
episode again. They gain Retry, which opens a fresh stream (through the
session refresh) from where the failed one stopped. A stall that lasts
twenty seconds now says so and offers the same, rather than showing
"Buffering…" for ever.

ios/ProtonStream/Player/PlayerView.swift

@@ -94,9 +94,14 @@ struct PlayerView: View {
                         // Dismissible, because the message is not always fatal
                         // — a subtitle track that failed to load leaves an
                         // episode that plays perfectly well behind it.
-                        Button("Dismiss", action: host.dismissProblem)
-                            .buttonStyle(.quiet)
-                            .tint(scheme.onErrorContainer)
+                        HStack {
+                            Button("Dismiss", action: host.dismissProblem)
+                                .buttonStyle(.quiet)
+                                .tint(scheme.onErrorContainer)
+                            Button("Retry", action: host.retry)
+                                .buttonStyle(.quiet)
+                                .tint(scheme.onErrorContainer)
+                        }
                     }
                     .textStyle(.bodyLarge)
                     .padding(16)