fix: resolve infinite loading spinner on Logs page

[Fizmatik]

Summary

The Logs page shows an infinite loading spinner and never displays any log entries. This regression was introduced when log watching was migrated from file-based polling to gRPC streams.

Root cause: BehaviorSubject was created without a seed value, so logController.stream never emitted an initial value. The UI stays in AsyncLoading state permanently, rendering SliverLoadingBodyPlaceholder (the spinner).

Changes

• Seed BehaviorSubject with empty list — ensures the stream always has an initial value for subscribers
• Yield current buffer immediately in watchLogs() so the UI transitions to AsyncData right away
• Add file-based fallback when gRPC core is not initialized — polls the log file directly (1s interval), matching the previous working implementation. Automatically stops when gRPC becomes available.
• Use List.of() at all emission points — prevents mutable list aliasing where the same list reference was shared between the stream, BehaviorSubject, and UI consumers
• Set cancelOnError: false and remove explicit cancel from onError handler — transient gRPC errors no longer permanently kill log collection
• Set proper Timestamp on file-sourced log entries to avoid displaying epoch dates
• Reset file position and notify logController on clearLogs()

Comparison with previous (working) implementation

The old ffi_singbox_service.dart used Watcher to poll the log file and always yielded an initial value:

Stream<List<String>> watchLogs(String path) async* {
  yield await _readLogFile(File(path));  // immediate yield
  yield* Watcher(path, ...).events.asyncMap(...);
}

The new gRPC-based implementation omitted both the initial yield and any fallback, causing the regression.

Test plan

• Open Logs page when VPN is disconnected — should show empty list, not spinner
• Open Logs page when VPN is connected — should show live logs
• Connect VPN while on Logs page — logs should start appearing
• Use level filter dropdown — should filter correctly
• Clear logs button — should clear the list
• Verify on both desktop and mobile