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/components/agents.js

@@ -285,19 +285,23 @@ export class AgentsPage extends LightElement {
         ${this._detail
           ? this._renderDetail()
           : html`
-            <div class="agents-page-header">
-              <h2 class="llm-page-title">${t('agents.title')}</h2>
-            </div>
-
-            <div class="agent-info-banner">
-              <div class="agent-info-banner-icon"><i class="bi bi-info-circle-fill"></i></div>
-              <div class="agent-info-banner-body">
-                <p class="mb-1">${unsafeHTML(t('agents.banner.title'))}</p>
-                <p class="mb-0">${unsafeHTML(t('agents.banner.text'))}</p>
+            <div class="page-header">
+              <div class="page-header-left">
+                <h2 class="page-header-title">${t('agents.title')}</h2>
               </div>
             </div>
 
-            ${this._renderList()}
+            <div class="agents-body">
+              <div class="agent-info-banner">
+                <div class="agent-info-banner-icon"><i class="bi bi-info-circle-fill"></i></div>
+                <div class="agent-info-banner-body">
+                  <p class="mb-1">${unsafeHTML(t('agents.banner.title'))}</p>
+                  <p class="mb-0">${unsafeHTML(t('agents.banner.text'))}</p>
+                </div>
+              </div>
+
+              ${this._renderList()}
+            </div>
           `
         }
       </div>