[ML 0.1/UI.04.8] - Wire message item click to legacy navigation handler

Blocked by

• #10775

Description

When a user taps a message item in the Compose MessageListScreen, the MessageItemEvent.OnMessageClick event is dispatched but silently dropped - no state transition, no side effect, no navigation occurs.

Connect MessageItemEvent.OnMessageClick to the legacy MessageListFragment.onMessageClicked() so that tapping a message opens the message detail / thread view, matching legacy behaviour.

Acceptance Criteria

• MessageItemEvent.OnMessageClick is handled in the state machine (SetupLoadedMessagesState).
• A MessageListEffect is emitted that triggers the legacy navigation path.
• The side effect or bridge invokes MessageListFragment's existing click handler (onMessageClicked / showThread).
• No new navigation logic is introduced - only wiring to existing handlers.
• Behaviour matches the legacy XML-based message list.

Out of Scope:

• Changing navigation architecture or introducing Compose Navigation.
• Modifying the legacy click handler logic.