Add HTML report generation with dark theme

Implements comprehensive HTML report generation from JSON scan data with Jinja2 templates. Reports feature a dark slate theme with summary dashboard, drift alerts, security warnings, and expandable service details.

Features:
- Dark theme HTML reports with slate/grey color scheme
- Summary dashboard: scan statistics, drift alerts, security warnings
- Site-by-site breakdown with IP grouping and status badges
- Expandable service details and SSL/TLS certificate information
- Visual badges: green (expected), red (unexpected), yellow (missing)
- UDP port handling: shows expected, unexpected, and missing UDP ports
- Screenshot links with relative paths for portability
- Optimized hover effects for table rows
- Standalone HTML output (no external dependencies)

Technical changes:
- Added src/report_generator.py: HTMLReportGenerator class with summary calculations
- Added templates/report_template.html: Jinja2 template for dynamic reports
- Added templates/report_mockup.html: Static mockup for design testing
- Updated requirements.txt: Added Jinja2==3.1.2
- Updated README.md: Added HTML report generation section with usage and features
- Updated CLAUDE.md: Added implementation details, usage guide, and troubleshooting

Usage:
  python3 src/report_generator.py output/scan_report.json

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

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

README.md

@@ -41,6 +41,13 @@ A dockerized network scanning tool that uses masscan for fast port discovery, nm
 
 ### Reporting & Output
 - **Machine-readable JSON output** format for easy post-processing
+- **HTML report generation**:
+  - Comprehensive HTML reports with dark theme for easy reading
+  - Summary dashboard with scan statistics, drift alerts, and security warnings
+  - Site-by-site breakdown with expandable service details
+  - Visual badges for expected vs. unexpected services
+  - SSL/TLS certificate details with expiration warnings
+  - One-click generation from JSON scan data
 - **Dockerized** for consistent execution environment and root privilege isolation
 - **Expected vs. Actual comparison** to identify infrastructure drift
 - Timestamped reports with complete scan duration metrics
@@ -269,17 +276,87 @@ Screenshots are captured on a best-effort basis:
 - Failed screenshots are logged but don't stop the scan
 - Services without screenshots simply omit the `screenshot` field in JSON output
 
+## HTML Report Generation
+
+SneakyScanner can generate comprehensive HTML reports from JSON scan data, providing an easy-to-read visual interface for analyzing scan results.
+
+### Generating Reports
+
+After completing a scan, generate an HTML report from the JSON output:
+
+```bash
+# Generate HTML report (creates report in same directory as JSON)
+python3 src/report_generator.py output/scan_report_20251113_175235.json
+
+# Specify custom output path
+python3 src/report_generator.py output/scan_report.json /path/to/custom_report.html
+```
+
+### Report Features
+
+The generated HTML report includes:
+
+**Summary Dashboard**:
+- **Scan Statistics**: Total IPs scanned, TCP/UDP ports found, services identified, web services, screenshots captured
+- **Drift Alerts**: Unexpected TCP/UDP ports, missing expected services, new services detected
+- **Security Warnings**: Expiring certificates (<30 days), weak TLS versions (1.0/1.1), self-signed certificates, high port services (>10000)
+
+**Site-by-Site Breakdown**:
+- Organized by logical site grouping from configuration
+- Per-IP sections with status badges (ping, port drift summary)
+- Service tables with expandable details (click any row to expand)
+- Visual badges: green (expected), red (unexpected), yellow (missing/warning)
+
+**Service Details** (click to expand):
+- Product name, version, extra information, OS type
+- HTTP/HTTPS protocol detection
+- Screenshot links for web services
+- SSL/TLS certificate details (expandable):
+  - Subject, issuer, validity dates, serial number
+  - Days until expiration with color-coded warnings
+  - Subject Alternative Names (SANs)
+  - TLS version support (1.0, 1.1, 1.2, 1.3) with cipher suites
+  - Weak TLS and self-signed certificate warnings
+
+**UDP Port Handling**:
+- Expected UDP ports shown with green "Expected" badge
+- Unexpected UDP ports shown with red "Unexpected" badge
+- Missing expected UDP ports shown with yellow "Missing" badge
+- Note: Service detection not available for UDP (nmap limitation)
+
+**Design**:
+- Dark theme with slate/grey color scheme for comfortable reading
+- Responsive layout works on different screen sizes
+- No external dependencies - single HTML file
+- Minimal JavaScript for expand/collapse functionality
+- Optimized hover effects for table rows
+
+### Report Output
+
+The HTML report is a standalone file that can be:
+- Opened directly in any web browser (Chrome, Firefox, Safari, Edge)
+- Shared via email or file transfer
+- Archived for compliance or historical comparison
+- Viewed without an internet connection or web server
+
+Screenshot links in the report are relative paths, so keep the report and screenshot directory together.
+
 ## Project Structure
 
 ```
 SneakyScanner/
 ├── src/
 │   ├── scanner.py           # Main scanner application
-│   └── screenshot_capture.py # Webpage screenshot capture module
+│   ├── screenshot_capture.py # Webpage screenshot capture module
+│   └── report_generator.py  # HTML report generation module
+├── templates/
+│   ├── report_template.html # Jinja2 template for HTML reports
+│   └── report_mockup.html   # Static mockup for design testing
 ├── configs/
 │   └── example-site.yaml    # Example configuration
 ├── output/                  # Scan results
 │   ├── scan_report_*.json   # JSON reports with timestamps
+│   ├── scan_report_*.html   # HTML reports (generated from JSON)
 │   └── scan_report_*_screenshots/  # Screenshot directories
 ├── Dockerfile
 ├── docker-compose.yml
@@ -298,12 +375,12 @@ Only use this tool on networks you own or have explicit authorization to scan. U
 
 ## Future Enhancements
 
-- **HTML Report Generation**: Build comprehensive HTML reports from JSON output with:
-  - Service details and SSL/TLS information
-  - Visual comparison of expected vs. actual results
-  - Certificate expiration warnings
-  - TLS version compliance reports
-  - Embedded webpage screenshots
+- **Enhanced HTML Reports**:
+  - Sortable/filterable service tables with JavaScript
+  - Interactive charts and graphs for trends
+  - Timeline view of scan history
+  - Embedded screenshot thumbnails (currently links only)
+  - Export to PDF capability
 - **Comparison Reports**: Generate diff reports showing changes between scans
 - **Email Notifications**: Alert on unexpected changes or certificate expirations
 - **Scheduled Scanning**: Automated periodic scans with cron integration