feat: persistent storage for Deno static sites

- Add storage_enabled and storage_limit_mb columns to static_sites.
- Create/attach Docker volumes (tinyforge-site-{name}-data) for Deno
  sites with storage enabled, mounted at /app/data.
- Grant --allow-write=/app/data in Deno container CMD.
- Add storage usage API endpoint (GET /api/sites/{id}/storage).
- Show storage section in site detail page with usage bar.
- Add storage toggle and limit field to new site wizard.
- Use ConfirmDialog for secret deletion instead of inline delete.

internal/api/router.go

@@ -255,6 +255,7 @@ func (s *Server) Router() chi.Router {
 			r.Route("/sites/{id}", func(r chi.Router) {
 				r.Get("/", s.getStaticSite)
 				r.Get("/secrets", s.listStaticSiteSecrets)
+				r.Get("/storage", s.getStaticSiteStorage)
 
 				// Admin-only mutations.
 				r.Group(func(r chi.Router) {