feat: Implement interactive HTML reports with service graph visualization

[nik-kale]

Summary

This PR transforms HTML report generation from simple markdown-wrapped text into a fully interactive, visually appealing report with service graph visualization, collapsible sections, and modern UI design.

Changes

• Complete rewrite of generate_html_report() function
• Added SVG-based service graph visualization with:
  • Circular force-directed layout
  • Color-coded root cause highlighting (red)
  • Dependency arrows showing service relationships
  • Incident markers on affected services
• Implemented interactive timeline with color-coded incidents
• Created collapsible evidence and remediation sections
• Added modern, responsive CSS with:
  • Gradient headers
  • Stat cards for key metrics
  • Professional color scheme
  • Hover effects and transitions
• Implemented helper functions:
  • _generate_service_graph_svg() for graph visualization
  • _generate_timeline_html() for incident timeline
  • _generate_candidates_html() for root cause candidates
• Added JavaScript for interactivity (collapsible sections)
• All assets inline (no external dependencies)

Type of Change

• New feature (non-breaking change adding functionality)

Features

• Self-contained HTML (no external CSS/JS dependencies)
• Responsive design works on desktop and mobile
• SVG service graph shows topology and dependencies
• Interactive collapsible sections to reduce clutter
• Visual hierarchy with color-coded severity
• Copy-to-clipboard functionality (via button)
• Professional gradient design

Visual Improvements

• Executive summary in highlighted box
• Service graph with circular layout
• Stat cards showing key metrics
• Color-coded root causes (red) vs normal services (blue)
• Timeline with formatted timestamps
• Numbered remediation steps with circular badges
• Collapsible evidence sections

Checklist

• Code follows project style guidelines
• Self-review completed
• No new warnings introduced
• All functionality self-contained

Related Issues

Implements Feature #7 from feature roadmap

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