feat(editor): add source line numbers

[murongg]

Summary

• add optional CodeMirror line numbers to Source mode and the source side of split view
• persist the preference through Editor settings and add localized labels for every supported locale
• update the line-number gutter without recreating the editor; visual preview remains unchanged

Validation

• pnpm test
• pnpm typecheck:test
• pnpm build
• manually enabled line numbers in the local web app and checked Source and Preview + Source layouts, including overflow and console errors

Risk

Low. The preference defaults to off and uses CodeMirror built-in line numbers without adding a dependency.

Closes #537

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jul 17, 2026 1:19pm