feat: add Chart.js and Flatpickr with CDN fallback strategy

Add optional libraries with local fallback for offline development:

Vendor files added:
- static/shared/js/vendor/chart.umd.min.js (Chart.js 4.4.1)
- static/shared/js/vendor/flatpickr.min.js (Flatpickr 4.6.13)
- static/shared/css/vendor/flatpickr.min.css

Template updates:
- admin/base.html: Add optional blocks for chartjs_script, flatpickr_script, flatpickr_css
- shared/includes/optional-libs.html: CDN fallback loaders for both libraries

Usage in page templates:
  {% block flatpickr_css %}
    {% from 'shared/includes/optional-libs.html' import flatpickr_css_loader %}
    {{ flatpickr_css_loader() }}
  {% endblock %}

  {% block chartjs_script %}
    {% from 'shared/includes/optional-libs.html' import chartjs_loader %}
    {{ chartjs_loader() }}
  {% endblock %}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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