Add default content security policy

[rdhar]

This pull request adds a Content Security Policy (CSP) configuration to the Astro site settings to improve site security per Astro v6.0 (see docs).

Security enhancements:

• Added a security.csp configuration with a directive of ["default-src 'self'"] in astro.config.mjs to restrict resource loading to the same origin.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Switch Markdown highlighting before turning on CSP

Enabling security.csp here introduces a user-visible regression for code blocks because this repo still uses Astro’s default Markdown highlighter (Shiki) and Astro 6 explicitly notes Shiki is incompatible with CSP due to inline styles. Since no markdown.syntaxHighlight override is configured, pages with fenced code blocks (for example src/content/blog/markdown-style-guide.md) can lose syntax-highlighting styles under this policy unless highlighting is migrated (e.g., to Prism) or CSP handling is adjusted.

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