mobile: don't downgrade wss:// relay URLs to plaintext ws://

RelayConfig.wsUrl derived the WebSocket scheme with
`uri.scheme == 'https' ? 'wss' : 'ws'`, mapping every non-https scheme
(including wss) to plaintext ws, so a base URL already using wss:// was
silently downgraded to ws://.

Invite-joined communities store their relay as wss://, so invited members'
connections lost transport encryption and failed outright against TLS-only
relays, making the community unusable for them.

Map the scheme explicitly (https/wss -> wss, http/ws -> ws, other schemes
pass through unchanged); only the wss -> ws downgrade changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Max Lampert <maxwell@squareup.com>

mobile/lib/shared/relay/relay_provider.dart

@@ -17,10 +17,18 @@ class RelayConfig {
 
   const RelayConfig({required this.baseUrl, this.nsec});
 
-  /// Derive the websocket URL from the HTTP base URL.
+  /// Derive the websocket URL from the base URL.
+  ///
+  /// A base that is already `ws`/`wss` (e.g. an invite-joined community's
+  /// `wss://` relay) passes through unchanged — mapping it to `ws` would be a
+  /// silent TLS downgrade that also fails against TLS-only relays.
   String get wsUrl {
     final uri = Uri.parse(baseUrl);
-    final scheme = uri.scheme == 'https' ? 'wss' : 'ws';
+    final scheme = switch (uri.scheme) {
+      'https' || 'wss' => 'wss',
+      'http' || 'ws' => 'ws',
+      _ => uri.scheme,
+    };
     return uri.replace(scheme: scheme).toString();
   }
 }

mobile/test/shared/relay/relay_provider_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:buzz/shared/relay/relay_provider.dart';
+
+void main() {
+  group('RelayConfig.wsUrl', () {
+    test('maps http-family schemes to their websocket equivalent', () {
+      expect(
+        const RelayConfig(baseUrl: 'https://relay.example').wsUrl,
+        'wss://relay.example',
+      );
+      expect(
+        const RelayConfig(baseUrl: 'http://localhost:3000').wsUrl,
+        'ws://localhost:3000',
+      );
+    });
+
+    test('passes ws-family schemes through without downgrading', () {
+      // An invite-joined community stores relayUrl as wss://…; downgrading it to
+      // plaintext ws:// is a TLS downgrade and fails against TLS-only relays.
+      expect(
+        const RelayConfig(baseUrl: 'wss://relay.example').wsUrl,
+        'wss://relay.example',
+      );
+      expect(
+        const RelayConfig(baseUrl: 'ws://localhost:3000').wsUrl,
+        'ws://localhost:3000',
+      );
+    });
+  });
+}