Redesign issues page with sidebar layout and sticky groups

[stylessh]

Summary

• Restructured the issues page to match the pulls page layout: two-column grid with a sticky sidebar containing metric cards and a main content area with grouped issue lists
• Created a dedicated IssueRow component with state-colored icons, author avatars, relative timestamps, and comment counts
• Added sticky group headers with shadow-on-stick behavior and scroll tracking

Test plan

• Verify the issues page renders with sidebar metric cards and grouped issue lists
• Confirm sticky headers activate shadow when scrolling within groups
• Check metric cards link to correct anchor sections and disable when count is 0
• Test responsive layout collapses to single column below xl breakpoint