refactor(document): extract large match arms into helper methods

Part of #2740 - Code quality refactors.

This commit addresses the 'Match statements' section of the tracking
issue by extracting three large match arms from process_message() into
dedicated helper methods:

- handle_group_selected_layers: Handles layer grouping in both artboard
  and non-artboard workflows (~50 lines extracted)
- handle_move_selected_layers_to: Handles layer movement with proper
  transform preservation (~85 lines extracted)
- handle_nudge_selected_layers: Handles keyboard nudging with optional
  resize mode (~75 lines extracted)

The main match statement in DocumentMessageHandler is now ~210 lines
shorter and more readable. Logic and behavior remain unchanged.

Author: theaniketgiri