feat: Add PDF Export for Executive Reports

[VelkaRepo]

Objective
Expand the reporting module to generate downloadable PDF reports for executive summaries and audit compliance.

Implementation Plan

• Utilize jinja2 for HTML templating (already implemented in v2.0).
• Integrate WeasyPrint or pdfkit to render the HTML + CSS into a finalized PDF document.
• Include visual charts (Pie Chart of Threat Distribution) generated by matplotlib.

Acceptance Criteria

• Add --format pdf to the CLI arguments.
• Generated PDF must include the HashShield branding header.
• PDF must be searchable (text-based, not an image).