fix(search): center and highlight matched content

[Albert-PZY]

Summary

• center the active find or replacement result in the viewport
• highlight all matches, the active match, and completed replacements
• reveal hidden Markdown source matches and outline embedded node-view matches
• clear transient highlights when search closes

Verification

• pnpm test tests/editor-shell.test.ts
• pnpm verify
• Playwright MCP: long-distance find, next-match navigation, embedded code match, replacement, and cleanup