feat: server config, token client, TLS pinning, connection test (Phase 1)

ServerConfig parses https base URL and derives wss livekit URL;
TokenClient POSTs /token with clean 503/offline error states;
TLSPinning holds optional cert SHA-384 pins (empty = system trust);
Settings gains a Connection test screen proving the endpoint works.

Sources/Features/Settings/SettingsView.swift

@@ -18,6 +18,11 @@ struct SettingsView: View {
             Section("Interaction") {
                 Toggle("Text mode", isOn: $settings.textMode)
             }
+            Section {
+                NavigationLink("Connection test") {
+                    ConnectionTestView(settings: settings)
+                }
+            }
         }
         .navigationTitle("Settings")
     }