web: unify page headers into one shared page-header bar

Every full-page view now renders the same sticky top bar (back button,
title, right-side actions) from the new web/css/page-header.css,
replacing a dozen per-page duplicates (.page-panel-header,
.project-page-header, .task-page-header, .llm-page-header, .um-header,
.apr-header, .llmr-header, .pv-header, .sa-header, .config-page-header,
.agents-page-header). Pages that padded the whole container (config,
agents, system-agents, llm-requests, models-hub) move that padding into
a body wrapper so the bar sits flush and stays pinned on scroll.
Back buttons are standardized to the icon-only .page-header-back.

web/css/users-roles.css

@@ -21,18 +21,6 @@
   margin: 0;
 }
 
-.um-header-right {
-  margin-left: auto;
-  display: flex;
-  align-items: center;
-  gap: 14px;
-}
-
-.um-header-count {
-  font-size: .82rem;
-  color: var(--placeholder-color);
-}
-
 .um-table-wrap {
   padding: 0 24px 24px;
 }