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/agents.css

@@ -5,7 +5,6 @@ agents-page {
   flex-direction: column;
   flex: 1;
   overflow-y: auto;
-  padding: 1.5rem 2rem;
   background: var(--bs-body-bg);
 }
 
@@ -15,11 +14,8 @@ agents-page {
   width: 100%;
 }
 
-.agents-page-header {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  margin-bottom: 0.75rem;
+.agents-body {
+  padding: 1.5rem 2rem;
 }
 
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