C6: Migrate /pipelines list + detail to unified design system

[nextlevelshit]

Parent

Epic #1624
Depends on: #1634 (C5)

Scope

Migrate /pipelines and /pipelines/{name} from sidebar layout to standalone pages.

Acceptance criteria

• /pipelines renders as standalone with unified nav
• /pipelines/{name} renders as standalone with unified nav
• Pipeline cards/rows use design system classes
• Visual smoke in chromium

Files touched

• internal/webui/templates/pipelines.html (rewrite)
• internal/webui/templates/pipeline_detail.html (rewrite)
• internal/webui/embed.go
• Handler files for ExecuteTemplate -> Execute