Refactor oversized UI modules into maintainable submodule structure

[dasarne]

Background

The following UI modules have grown significantly and now contain multiple responsibilities:

• src/ui/editor_panel.py
• src/ui/canvas_panel.py
• src/ui/main_window.py

This already conflicts with the architectural guidance documented in AGENTS.md:

• separation of concerns
• reusable services
• maintainable UI structure
• rendering isolation
• testability

Goals

Refactor the UI architecture into smaller focused modules while preserving all existing functionality and UX behavior.

Proposed Structure

Editor

src/ui/editor/

• editor_widget.py
• editor_selection.py
• editor_highlighting.py
• editor_comments.py
• editor_undo.py
• editor_search.py

Canvas

src/ui/canvas/

• canvas_widget.py
• canvas_rendering.py
• canvas_selection.py
• canvas_navigation.py
• canvas_layers.py
• canvas_cache.py

Main Window

src/ui/mainwindow/

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

Requirements

• Preserve all existing UX behavior
• Preserve editor responsiveness
• Preserve rendering performance
• Avoid unnecessary reparsing or allocations
• Keep rendering logic isolated from application logic
• Avoid introducing hidden global state
• Keep tests green during the migration
• Refactor incrementally in reviewable steps

Acceptance Criteria

• Large monolithic UI files reduced substantially
• Responsibilities clearly separated
• Existing tests still pass
• No UX regressions
• Easier navigation and maintainability
• Foundation prepared for stricter typing and future UI tests