Expand Flutter test coverage to include chat flow, approval flow, and settings

[neooriginal]

This issue is AI generated.

Problem

The Flutter test suite currently has 6 test files:

• test/flutter/unit/backend_client_test.dart
• test/flutter/unit/mcp_server_item_test.dart
• test/flutter/unit/password_strength_test.dart
• test/flutter/unit/task_item_test.dart
• test/flutter/widget/auth_form_test.dart
• test/flutter/widget/splash_view_test.dart

The following critical user flows have zero test coverage:

• Sending a chat message and streaming the response
• The tool approval flow (approval request arrives → sheet shown → user decides → result sent)
• Settings save/discard lifecycle
• Navigation between panels
• WebSocket connect/disconnect/reconnect state transitions

This makes refactoring of main_controller.dart and the panel widgets risky and regressions hard to catch.

Required change

Add widget and unit tests for at minimum:

• ChatPanel: send message, streaming message display, tool event items appearing, scroll-to-bottom behaviour.
• SecurityPanel + _ToolApprovalSheet: approval request received, approve/deny, submission error handling, concurrent approval queuing.
• SettingsPanel: settings load, save, unsaved-changes guard.
• NeoAgentController: WebSocket state transitions, pendingApproval queue management.

Acceptance criteria

• Test files exist for each of the listed panels and flows.
• Coverage includes the happy path and at least one error/edge case per flow.
• All new tests pass in CI (npm run flutter:test).