fix(editor): support line breaks in visual table cells

[murongg]

Summary

• let Shift+Enter insert a persisted <br> in visual table cells while keeping focus on the new visual line
• preserve the existing plain Enter behavior that finishes cell editing
• distinguish authored line breaks from browser-owned empty-cell placeholders, including a cell whose only content is <br>

Why

Visual table cells handled every Enter keypress as a request to leave editing, so the existing source-mode Shift+Enter behavior was unavailable in preview mode. This predates and is independent of #630.

Validation

• pnpm --filter @markra/editor test — 33 files, 387 tests passed
• pnpm --filter @markra/app test -- MarkdownExportDocument.test.tsx — 9 tests passed
• pnpm --filter @markra/editor build — passed
• pnpm --filter @markra/editor typecheck:test — passed
• git diff --check — passed
• browser QA: inserted a line break, continued typing on the next visual line, and confirmed the source became First<br>Second

Risk

• scoped to Enter handling and exact <br> rendering/serialization inside visual table cells
• non-break HTML remains literal, and plain Enter behavior is unchanged

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview	Aug 1, 2026 4:35pm