feat(app): add file tree multi-selection

[murongg]

Summary

• add Cmd/Ctrl and Shift multi-selection for sidebar markdown files
• support selected-file batch delete with plural confirmation copy
• disable single-file-only context menu actions during multi-select
• clear multi-selection when clicking outside file rows

Validation

• pnpm --filter @markra/app test
• pnpm exec vitest run src/runtime/context-menu-items.test.ts src/components/MarkdownFileTreeDrawer.test.tsx
• pnpm --filter @markra/app build
• pnpm --filter @markra/app typecheck:test
• pnpm --filter @markra/shared test -- i18n
• pnpm --filter @markra/shared build
• pnpm --filter @markra/desktop exec tsc -p tsconfig.app.json --noEmit
• git diff --check

Closes #364

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jun 21, 2026 9:10am