style(ui): adopt Liquid Glass on opaque controls

Swap hardcoded systemGray/Color.blue pill and button fills for the iOS 26
Liquid Glass APIs so controls read native under the new design system:
- BookmarkRow tag chips: glassEffect in a GlassEffectContainer
- Podcast speed button: glassEffect capsule
- Podcast retry + Onboarding Connect: glassProminent button style

Onboarding text-field fills left as systemGray6 (standard adaptive field
fill; glass behind editable text hurts legibility).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Views/OnboardingView.swift

@@ -89,10 +89,9 @@ struct OnboardingView: View {
                 }
                 .frame(maxWidth: .infinity)
                 .frame(height: 52)
-                .background(canConnect ? Color.primary : Color(.systemGray4))
-                .foregroundStyle(canConnect ? Color(UIColor.systemBackground) : Color(.systemGray2))
-                .clipShape(RoundedRectangle(cornerRadius: 14))
             }
+            .buttonStyle(.glassProminent)
+            .buttonBorderShape(.roundedRectangle(radius: 14))
             .disabled(!canConnect || isConnecting)
             .padding(.horizontal, 24)
             .padding(.top, 24)