Decompose main_window.py into workflow and UI orchestration modules

[dasarne]

Parent issue: #17

Goal

Refactor src/ui/main_window.py into smaller orchestration-focused modules while preserving all application workflows and UI behavior.

Current Problems

The file currently combines:

• menu construction
• actions
• dialogs
• file workflows
• status handling
• editor/canvas orchestration
• settings integration
• warning handling
• localization updates
• navigation glue logic

This results in:

• very large change surfaces
• difficult navigation
• difficult testing
• hidden coupling
• difficult typing and maintenance

Proposed Target Structure

src/ui/mainwindow/

• main_window.py
• menus.py
• actions.py
• statusbar.py
• file_workflows.py
• dialog_management.py

Architectural Goals

• Keep orchestration responsibilities explicit
• Avoid hidden cross-module coupling
• Keep business logic outside the main window
• Improve readability and testability
• Reduce merge conflicts for future work

Suggested Extraction Order

1. Status bar handling
2. Menu creation
3. QAction setup
4. Dialog management
5. File workflows
6. Final orchestration cleanup

Requirements

• Preserve all shortcuts
• Preserve workflow behavior
• Preserve editor/canvas synchronization
• Preserve localization behavior
• Keep tests green throughout migration

Acceptance Criteria

• main_window.py reduced substantially
• Responsibilities clearly separated
• Existing workflows preserved
• Existing tests still pass
• Reduced coupling and easier future maintenance