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/models-hub.css

@@ -1,10 +1,13 @@
 /* ── Models hub landing page ────────────────────────────────────────────────── */
 
 .models-hub {
-  padding: 2rem 2.5rem;
   width: 100%;
 }
 
+.models-hub-body {
+  padding: 2rem 2.5rem;
+}
+
 .models-hub-grid {
   display: grid;
   grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));