C7: Migrate /issues, /prs, /health to unified design system

[nextlevelshit]

Parent

Epic #1624
Depends on: #1634 (C5)

Scope

Batch-migrate the simpler list+detail pages: /issues, /issues/{number}, /prs, /prs/{number}, /health.

Acceptance criteria

• All five pages render as standalone with unified nav
• Tables/lists use w-list design system classes
• Status badges use badge-* classes
• Visual smoke in chromium

Files touched

• internal/webui/templates/issues.html + issue_detail.html
• internal/webui/templates/prs.html + pr_detail.html
• internal/webui/templates/health.html
• internal/webui/embed.go
• Handler files for ExecuteTemplate -> Execute