fix(security): update demo-video deps to fix GHSA-5c6j-r48x-rmvq (#105)

## Summary
Updates transitive dependency `serialize-javascript` in `demo-video/` to
fix GHSA-5c6j-r48x-rmvq (RCE via RegExp.flags and
Date.prototype.toISOString).

### Changes
- **demo-video/package-lock.json**: `npm audit fix` resolved the
vulnerability

### Context
`serialize-javascript@6.0.2` was pulled in via `@remotion/cli` →
`webpack` → `terser-webpack-plugin`. The vulnerability allows arbitrary
code execution through crafted RegExp objects when serialized output is
evaluated. While this is dev tooling only, keeping dependencies patched
is best practice.

Closes #101

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dorlugasigal