[Refactor] Migrate to official Flutter multi-window support

[mikai233]

[Refactor] Migrate to official Flutter multi-window support and replace desktop_multi_window

Pre-flight checks

• I searched existing issues for duplicates.
• This is not a general support question.

Problem statement

The current multi-window implementation relies on the desktop_multi_window plugin, which presents several significant challenges:

1. Linux Instability: The plugin causes crashes on Linux (likely due to EGL/GLX context issues with multiple engines), forcing the project to disable multi-window support on that platform.
2. Multi-Engine Overhead: desktop_multi_window spawns a full Flutter Engine for every window. This is resource-intensive.
3. Storage/Hive File Locking: Because each engine runs in a separate process/thread context, Hive's file-based locking prevents multiple windows from accessing the same database. This forces a complex architecture where only the main window has DB access, and others must use a memory-cached "remote" storage that syncs via method channels.
4. Complex Data Sync: Synchronizing state (settings, emulator state, etc.) between multiple engines is error-prone and requires significant boilerplate code (see app_storage.dart and app_data_sync.dart).

Proposed solution

Refactor the multi-window logic to use official Flutter Multi-View / Multi-Window support once it matures (expected targeted focus in 2026).

Planned Changes

• Remove desktop_multi_window: Deprecate and eventually remove the third-party plugin.
• Unified Flutter Engine: Leverage Flutter's native multi-view architecture which allows multiple windows to share a single Flutter Engine.
• Simplify Persistence: Revert to a unified Hive storage model where all "views" can safely access the same data context without multi-engine locking issues.
• Unified State Management: Simplify Riverpod/State management as all windows will share the same Dart isolate.

Alternatives considered

• Stick with desktop_multi_window and try to fix Linux stability (unlikely to be worth the effort compared to waiting for first-party support).
• Use a different multi-engine IPC solution (doesn't solve the memory overhead or the fundamental multi-engine complexity).

Scope

• Frontend (Flutter)
• Build / CI / tooling

Additional context

• Official Flutter Tracking Issue: Flutter Multi-Window / Multi-View Support #142845
• Relevant Labels: topic: multi-window, a: desktop
• Current affected files:
  • lib/platform/desktop_window_manager.dart
  • lib/persistence/app_storage.dart
  • lib/windows/app_data_sync.dart