Sessions leave redacted logs; viewers wait for a member relay's source before opening it

Logs: one redaction pass (query strings to names, member tokens,
passwords, cookies, auth headers) for every line; daily files with size
and age caps in the app (hosted server included) and the dedicated
server (--log-dir); swaps, relay channels, per-member /r /m /j outcomes,
server fetches, mpv load errors, yt-dlp runs. Settings > About: Copy
diagnostics, Open logs folder.

Bilibili through someone else: the viewer opened /r/ before the source
dialled in; the server holds that request and mpv gives up after 5 s
("could not be opened"). Viewers now open a member relay once it is
live, the putter starts its source before its own player, and track
sizes are read in parallel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/platform/app_log.dart

@@ -0,0 +1,98 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
+import 'package:sync_relay_core/core.dart';
+
+import '../playback/yt_dlp_app.dart';
+import '../screens/home/ui.dart' show appName, displayVersion;
+import '../state/session_state.dart';
+
+// The app's log on disk (<app support>/logs/app-YYYY-MM-DD.log; core's
+// logging.dart writes, rotates and redacts it), debugPrint routed into it,
+// and the "Copy diagnostics" text of Settings → About.
+
+final _appLog = createLogger('app');
+
+/// The logs folder, once [startAppLogging] ran.
+String? appLogDir;
+
+/// Opens the log files and routes `debugPrint` and uncaught errors into
+/// the log. Needs Flutter's binding (path_provider).
+Future<void> startAppLogging() async {
+  try {
+    final base = await getApplicationSupportDirectory();
+    final dir = '${base.path}${Platform.pathSeparator}logs';
+    if (startLogFiles(dir) != null) appLogDir = dir;
+  } catch (e) {
+    _appLog.warning('no log files: $e');
+  }
+  // Everything the app prints for diagnosis lands in the file too (the
+  // log's console output replaces the print).
+  debugPrint = (String? message, {int? wrapWidth}) {
+    if (message == null || message.isEmpty) return;
+    final bad = RegExp(r'fail|error|crash|exception', caseSensitive: false).hasMatch(message);
+    bad ? _appLog.warning(message) : _appLog.info(message);
+  };
+  PlatformDispatcher.instance.onError = (error, stack) {
+    _appLog.severe('uncaught: $error', error, stack);
+    return false;
+  };
+  _appLog.info('$appName ${displayVersion()} on ${Platform.operatingSystem} '
+      '${Platform.operatingSystemVersion}, pid $pid, Dart ${Platform.version.split(' ').first}');
+}
+
+/// The text "Copy diagnostics" gives: versions, then the last [since] of
+/// the app log (its hosted server's records included), already redacted.
+Future<String> diagnosticsText({Duration since = const Duration(hours: 6)}) async {
+  final b = StringBuffer()
+    ..writeln('$appName ${displayVersion()}')
+    ..writeln('OS: ${Platform.operatingSystem} ${Platform.operatingSystemVersion}')
+    ..writeln('Dart: ${Platform.version.split(' ').first}');
+  try {
+    final mpv = await SessionState.instance.coordinator.playback.mpvVersion().timeout(const Duration(seconds: 2));
+    b.writeln('mpv: ${mpv ?? 'no player yet'}');
+  } catch (_) {
+    b.writeln('mpv: unknown');
+  }
+  try {
+    final y = await AppYtDlp.instance.ytDlp.timeout(const Duration(seconds: 2));
+    final cookies = y.cookies.read(count: false);
+    b.writeln('yt-dlp: ${y.version ?? 'not ready'}${y.lastError == null ? '' : ' (${y.lastError})'}, '
+        'cookies: ${cookies == null ? 'none' : 'set'}');
+  } catch (_) {
+    b.writeln('yt-dlp: unknown');
+  }
+  final files = logFiles;
+  b.writeln('Log: ${files?.currentPath ?? 'none (console only)'}');
+  b.writeln('--- last ${since.inHours} h of the log ---');
+  b.write(files?.recent(since) ?? '');
+  return redactLog(b.toString());
+}
+
+/// Saves [text] next to the logs as `diagnostics-<time>.txt`; its path,
+/// or null when it could not be written.
+Future<String?> saveDiagnostics(String text) async {
+  final dir = appLogDir;
+  if (dir == null) return null;
+  final t = DateTime.now();
+  String two(int n) => n.toString().padLeft(2, '0');
+  final path = '$dir${Platform.pathSeparator}diagnostics-${t.year}${two(t.month)}${two(t.day)}-'
+      '${two(t.hour)}${two(t.minute)}${two(t.second)}.txt';
+  try {
+    // Older ones go after the log files' week.
+    for (final e in Directory(dir).listSync()) {
+      if (e is File &&
+          e.uri.pathSegments.last.startsWith('diagnostics-') &&
+          t.difference(e.statSync().modified) > const Duration(days: 7)) {
+        e.deleteSync();
+      }
+    }
+    await File(path).writeAsString(text);
+    return path;
+  } catch (e) {
+    _appLog.warning('cannot save diagnostics: $e');
+    return null;
+  }
+}