Include scan duration/completion time in scan metadata and reports

[techmore]

Implement comprehensive scan duration tracking and enhanced summary banner with key metrics.

Combined Requirements (Issues #25 + #27 + UI Cleanup):

Duration Tracking:

• Record scan start and end times in metadata.json
• Calculate and store scan duration (seconds + formatted)
• Include duration in HTML/PDF reports
• Display duration in scan history and UI

Enhanced Summary Banner:

• Create prominent post-scan banner with comprehensive metrics
• Display: hosts discovered, subnet size, average ping times, CVE/exploit counts, scan duration
• Use visual indicators for critical information
• Position banner after scan completion with auto-hide option

UI Cleanup - Recovered Scan Card:

• Remove 'Run New Scan' button/option from the historical data recovery banner
• The recovered scan card should only display historical data without scan initiation options
• Maintain clear distinction between historical view and active scanning

Implementation Details:

• Modify save_scan_metadata() in app.py to accept start/end times and calculate duration
• Add duration fields to metadata JSON structure
• Create new Socket.IO event for scan summary data
• Implement banner HTML/CSS in templates/index.html
• Add statistics calculation from nmap XML results
• Update report generation to include duration in headers
• Remove scan initiation elements from historical data banner

Current State:

• Duration calculated but not saved in metadata
• Basic feedback messages only, no comprehensive banner
• Reports lack duration information
• Historical scan card may include inappropriate scan options
• No post-scan summary with key metrics

Benefits:

• Complete scan performance tracking
• Immediate user insights post-scan
• Better historical analysis capabilities
• Cleaner UI with appropriate historical vs active scan separation
• Enhanced user experience with comprehensive summaries

Location: app.py (metadata + stats calc), templates/index.html (banner + historical card cleanup), report generation functions.