feat: operate on and move selected block ranges

[Azganoth]

Summary

• Make selected block ranges operate as structural content for copy, cut, delete, typing, and paste, preserving nested list and blockquote structure and the valid remembered text caret after deletion.
• Add sibling movement through Alt+Up, Alt+Down, context popup commands, and handle dragging. All movement paths share boundary validation and preserve selection direction for Undo and Redo.
• Update the editor specification, command reference, changelog, and focused coverage for nested and cross-parent ranges.

Related Issue

Refs #482. Native OS clipboard transport remains to be manually verified before the issue is complete.

Verification

• pnpm vitest run src/features/editor/plugins/blockSelectionInteraction.test.ts src/features/editor/plugins/blockSelectionOperations.test.ts covers structural deletion, movement, drag cancellation, replacement, nested selections, and history.
• pnpm check:frontend passed formatting, TypeScript, lint, production build, and the full frontend test and coverage suite.
• pnpm build:e2e:desktop and pnpm test:e2e:desktop:run --scenario block-selection passed in the assembled Windows app, covering popup movement, keyboard movement, Undo, and pointer drag.
• The pre-commit and pre-push hooks passed.

Not verified: Native OS clipboard transport between Leafdown and other desktop applications. Clipboard formats and replacement behavior are covered by editor tests, but the system clipboard was not changed during this pass.