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/system-agents.css

@@ -9,29 +9,11 @@
   flex: 1;
   min-height: 0;
   width: 100%;
-  padding: 1.5rem;
   overflow-y: auto;
 }
-.sa-header {
-  display: flex;
-  align-items: baseline;
-  gap: 0.75rem;
-  margin-bottom: 0.35rem;
+.sa-body {
+  padding: 1.5rem;
 }
-.sa-title {
-  font-size: 1.2rem;
-  font-weight: 600;
-  margin: 0;
-}
-.sa-total-badge {
-  font-size: 0.75rem;
-  color: var(--bs-secondary-color);
-  background: var(--bs-tertiary-bg);
-  border: 1px solid var(--bs-border-color);
-  border-radius: 1rem;
-  padding: 0.1rem 0.6rem;
-}
-.sa-refresh-btn { margin-left: auto; }
 .sa-subtitle {
   font-size: 0.85rem;
   color: var(--bs-secondary-color);