feat: add total count column to issues listing page

[TeeLusk]

Summary

• Adds a new 'Total' column to the issues listing page showing the sum of errors in the last 24 hours
• Reuses existing issue count data with a simple reduce operation for optimal performance
• Matches existing styling patterns and responsive layout design
• Positions the column between the histogram and time columns for logical information flow

Implementation Details

• Added COL_TOTAL_WIDTH constant (80px) to maintain consistent column sizing
• Added totalCount computed memo using counts().reduce((sum, item) => sum + item.count, 0)
• Uses existing IssueCountStore.forIssue.watch() data source - no additional API calls
• Applied consistent text styling (color="secondary", weight="medium") to highlight the total
• Added toLocaleString() formatting for better number readability

This enhancement provides users with a quick overview of issue frequency alongside the detailed histogram breakdown.