feat(app): add contextual multi-selection

[nmrtist]

Summary

• add contextual Select All and Deselect All behavior based on the most recently used Canvas, Layers, Data, or object-selection context
• add standard desktop multi-selection across Canvas, Layers, and Data, including range selection, toggle selection, keyboard navigation, and Space
• add marquee selection for pages and worksheets on the infinite board
• support moving selected frames together with single-step undo and redo
• keep frame and Data selections synchronized using stable resource IDs
• align context menus, Escape handling, page duplication, commands, menus, automation targets, and dynamic command labels with multi-selection behavior
• update the English and Simplified Chinese documentation for shortcuts, data organization, and board layout

Testing

• cargo pr-check — all 7 stages passed
• full Rust test suite passed
• documentation build passed (73 pages)
• git diff --check passed
• no generated directories committed