refactor(app): drop Dart wrappers whose native handlers no longer exist

#17 removed the `setSessionExpiry` and `isVpnRuntimeAlive` MethodChannel
handlers from MainActivity, and #18 removed their last callers. The Dart
wrappers stayed behind.

Both swallow their errors, so a future caller would not fail loudly — it would
get a silent no-op from `setSessionExpiry` and a permanent
`processAlive: false` from `runtimeState()`, which previously read as "the
tunnel is dead". Deleting them keeps that trap out of the codebase.

`VpnRuntimeState` goes with them; runtime liveness now arrives through the
native status stream, which is generation-scoped. The battery-optimisation
methods are untouched and still used by background_reliability.dart.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019HT2x2JvJNg7XNsJubfHFf

lib/logic/power_settings.dart

@@ -41,53 +41,4 @@ abstract final class PowerSettings {
       return false;
     }
   }
-
-  /// Whether the VPN runtime is genuinely alive.
-  ///
-  /// The Xray service runs in a separate process, so nothing it stores in
-  /// statics is visible to us. This asks the OS instead: is our service process
-  /// running, and (for TUN) is there an active VPN transport?
-  static Future<VpnRuntimeState> runtimeState() async {
-    if (kIsWeb) return const VpnRuntimeState(processAlive: false, vpnTransport: false);
-    try {
-      final result = await _channel.invokeMapMethod<String, dynamic>(
-        'isVpnRuntimeAlive',
-      );
-      return VpnRuntimeState(
-        processAlive: result?['processAlive'] == true,
-        vpnTransport: result?['vpnTransport'] == true,
-      );
-    } catch (_) {
-      return const VpnRuntimeState(processAlive: false, vpnTransport: false);
-    }
-  }
-
-  /// Pushes the absolute session-expiry timestamp (epoch ms) to the VPN
-  /// service so it can tear the tunnel down even when the UI process is gone.
-  static Future<void> setSessionExpiry(int expiresAtMs) async {
-    if (kIsWeb) return;
-    try {
-      await _channel.invokeMethod<void>('setSessionExpiry', {
-        'expiresAtMs': expiresAtMs,
-      });
-    } catch (_) {
-      // Best-effort. The daemon also keeps its last persisted expiry.
-    }
-  }
-}
-
-@immutable
-class VpnRuntimeState {
-  final bool processAlive;
-  final bool vpnTransport;
-
-  const VpnRuntimeState({
-    required this.processAlive,
-    required this.vpnTransport,
-  });
-
-  /// TUN needs both a live service and a real VPN interface. Proxy mode has no
-  /// VPN interface at all, so the service process is the whole signal.
-  bool aliveFor({required bool tunMode}) =>
-      processAlive && (!tunMode || vpnTransport);
 }