fix(editor): restore horizontal rule selection

[murongg]

Summary

• Make horizontal rule clicks select the rule node directly, including rules below images and adjacent rules.
• Preserve surrounding-gap click protection while allowing the full rule hit target to select.
• Allow Backspace/Delete from empty paragraphs around horizontal rules to remove the blank paragraph and select the rule.

Why

Horizontal rules could lose their selectable hit target when adjacent to images or blank paragraphs, which also made nearby empty lines feel impossible to delete.

Validation

• pnpm vitest run src/components/MarkdownPaper.test.tsx
• pnpm --filter @markra/app typecheck:test
• git diff --check

Risk

• Low; scoped to horizontal-rule click handling and adjacent empty-paragraph keyboard behavior.

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jun 21, 2026 12:50pm