Upgrade @11ty/eleventy to v3 to fix markdown-it vulnerability

## Summary

There are 2 remaining moderate-severity npm vulnerabilities caused by `markdown-it` (13.0.0 - 14.1.0) which is a dependency of `@11ty/eleventy` v2.

- [GHSA-38c4-r59v-3vqw](https://github.com/advisories/GHSA-38c4-r59v-3vqw) — markdown-it ReDoS

## Required action

Upgrade `@11ty/eleventy` from v2 to v3 (`npm audit fix --force` will install v3.1.2). This is a **breaking change** that may require:

- Converting config from CJS to ESM
- Reviewing any custom plugins or filters for compatibility
- Testing the full site build after upgrade

## Context

All other vulnerabilities were resolved via `npm audit fix` (13 of 15 fixed).