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/plugin-catalog.js

@@ -95,8 +95,10 @@ export class PluginCatalogPage extends LightElement {
 
     return html`
       <div class="um-page">
-        <div class="um-header">
-          <h2 class="um-title"><i class="bi bi-puzzle-fill me-2"></i>${t('nav.plugins')}</h2>
+        <div class="page-header">
+          <div class="page-header-left">
+            <h2 class="page-header-title"><i class="bi bi-puzzle-fill me-2"></i>${t('nav.plugins')}</h2>
+          </div>
         </div>
 
         ${this._error ? html`