feat(den-web): add hidden plugins discovery page (#1472)

Introduce a new /plugins route under the org dashboard that lists plugins
(bundles of skills, hooks, MCP servers, agents, and commands) with a
list view, per-plugin detail view, and tabbed cross-cutting views for
all skills/hooks/MCPs.

- Reuses existing dashboard primitives: DashboardPageTemplate,
  UnderlineTabs, DenInput, PaperMeshGradient cards.
- Mirrors the skill-hubs list/detail patterns 1:1 for visual coherence.
- Introduces @tanstack/react-query (scoped to the dashboard subtree)
  with a mock queryFn so the data layer can swap to a real API later
  with a one-line change.
- Hidden by design: no sidebar nav entry. Reachable only via direct URL
  /o/<slug>/dashboard/plugins. Header title is mapped to 'Plugins' so
  navigating manually still feels coherent.

pnpm-lock.yaml

@@ -538,6 +538,9 @@ importers:
       '@paper-design/shaders-react':
         specifier: 0.0.72
         version: 0.0.72(@types/react@19.2.14)(react@19.2.4)
+      '@tanstack/react-query':
+        specifier: ^5.96.2
+        version: 5.96.2(react@19.2.4)
       lucide-react:
         specifier: ^0.577.0
         version: 0.577.0(react@19.2.4)