fix(editor): add trailing line after terminal blocks

[murongg]

Summary

• Add a trailing-line editor affordance after terminal container blocks such as tables, callouts, and lists.
• Insert a real paragraph only when the user clicks that trailing line, keeping loaded Markdown content unchanged until explicit editing.
• Cover tables, callouts, unordered lists, and ordered lists with a regression test.

Why

Terminal non-plain blocks at the end of the document had no top-level text position after them, so users could not click below the final block to create a new blank line.

Fixes #231

Validation

• pnpm --filter @markra/app exec vitest run src/components/MarkdownPaper.test.tsx passed
• pnpm --filter @markra/app test passed
• pnpm --filter @markra/editor test passed
• pnpm --filter @markra/app build passed
• pnpm --filter @markra/editor build passed
• git diff --check passed

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jun 10, 2026 2:35am