Add webview font fallbacks

[subhajitlucky]

Fixes #33.

Summary

• Adds explicit sans-serif fallbacks for UI text that uses the VS Code UI font variable.
• Adds editor/UI/monospace fallback chains for search result and preview code text.
• Prevents the webview from falling back to browser serif defaults when a configured VS Code font is unavailable.

Validation

• npm run compile
• git diff --check

Note: npm run lint still fails because the repository has no ESLint configuration file for the existing lint script.

[Copilot]

Pull request overview

This PR addresses issue #33 by ensuring the webview’s typography consistently falls back to sensible default font families when VS Code-provided font variables resolve to unavailable fonts on a given machine.

Changes:

• Add explicit sans-serif fallbacks for UI text using --vscode-font-family.
• Add editor/UI/monospace fallback chains for match text and preview code content using --vscode-editor-font-family.

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