Add topbar tab manager for PR and issue detail views

[stylessh]

Summary

• Adds a tab system to the dashboard topbar that tracks opened PR/issue detail views
• Tabs show type-appropriate icons with status-based colors, truncated title, and number
• Close button appears as an overlay on hover with a surface backdrop
• Tab list is horizontally scrollable with hidden scrollbar and gradient fade edges
• Tabs persist across hard refreshes via localStorage

Test plan

• Open several PR and issue detail views, verify tabs appear in the topbar
• Verify tab icon colors reflect PR/issue status (open, closed, merged, draft)
• Close tabs via hover X button, confirm active tab redirects to /
• Refresh the page, confirm tabs persist
• Add enough tabs to overflow, verify horizontal scroll and gradient edges work