C5: Migrate /runs list page to unified design system

[nextlevelshit]

Parent

Epic #1624
Depends on: #1633 (C3)

Scope

Migrate /runs from sidebar layout to standalone page using the unified design system (w-* CSS classes). This is the highest-traffic page in the webui.

Current state

• templates/runs.html — layout-based (sidebar), uses wr-* CSS classes, filter bar, infinite scroll, search, running section
• Handler: handlers_runs.go serves RunListPage data
• Complex template: filters, pipeline selector, top-level toggle, running section, infinite scroll

Target state

• Standalone page with w-nav, w-container, w-page-header, w-filterbar, w-list
• Running section preserved with collapsible behavior
• Infinite scroll preserved
• Client-side search preserved
• Status badges use badge-* design system classes
• No Tailwind dependency — pure style.css design system

Acceptance criteria

• /runs renders as standalone page with unified nav
• Filter bar (status, pipeline, search) works
• Running section collapsible
• Infinite scroll loads more runs
• Status badges colored correctly
• Visual smoke in chromium

Files touched

• internal/webui/templates/runs.html (rewrite)
• internal/webui/embed.go (move to standalonePageTemplates)
• internal/webui/handlers_runs.go (Execute instead of ExecuteTemplate)