Sanitize and expose statement HTML for math rendering; add client-side math renderer

[enlorik]

Motivation

• Preserve Codeforces pre-rendered math markup (e.g. tex-span, superscripts, subscripts) for client-side rendering while removing unsafe tags and attributes.
• Provide a safe HTML fragment alongside plain-text fields so the frontend can render math and richer markup without leaking scripts or event handlers.

Description

• Add a conservative sanitizer safeHtml to cfStatementParser.js that whitelists allowed tags and class prefixes and strips scripts, styles and other attributes, and return new HTML fields statementHtml, inputSpecificationHtml, and outputSpecificationHtml alongside existing text fields.
• Update cfStatementParser.test.js to expect the new HTML fields and to verify preservation of sanitized math markup.
• Add client-side utilities in src/utils/renderMath.js providing escapeHtml, normalizeLatex, renderMathExpression, renderMathInHtml, and textToHtml to convert LaTeX-like delimiters to safe HTML and to turn plain text into paragraph HTML.
• Integrate server-provided HTML into the workspace by adding StatementHtml in ProblemWorkspace.jsx which uses renderMathInHtml and textToHtml as a fallback, and wire the statement/input/output sections to use the new HTML fields.
• Add styling in ProblemWorkspace.css for .problem-workspace-stmt-text, .tex-span, .problem-workspace-math and related elements.
• Add src/utils/renderMath.test.js with unit tests for math rendering and update parser tests accordingly.

Testing

• Ran the updated parser unit tests in cfStatementParser.test.js under the test runner and they succeeded.
• Ran the new renderMath unit tests in src/utils/renderMath.test.js and they succeeded.
• The combined test suite (Vitest) completed without failures.

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Codex Task