* fix(app): restore diff copy actions on web
The canvas-backed diff owns selection without creating a DOM range, so browser context menus cannot expose copy actions themselves. Route keyboard and context-menu actions through the renderer's canonical source selection instead.
* fix(app): harden web diff copy actions
* fix(app): default split header selection to new pane
* fix(app): isolate nested diff context menus
* fix(app): select available split diff pane