v0.6.0 - Date Filtering and UX Improvements

Fixed
- Critical date boundary filtering bug preventing data leakage across reporting periods
- Timeline charts now use appropriate time columns (Start for CPU/GPU, Submit for jobs/users)
- Reports validation now prevents generation for incomplete/future periods
- User-friendly error messages for invalid date ranges and report parameters

Improved
- Daily period charts now show day of week (e.g., "Mon Jan 1")
- Clarified "Number of Submitted Jobs" labels to distinguish from started jobs
- Added visual validation warnings for invalid date ranges
- Removed top 10 limits when using color grouping for better data visibility
- Agent retry logic with increased attempts and timeout for reliability

Documentation
- Added comprehensive dashboard screenshot
- Simplified README structure