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-detail.js

@@ -198,12 +198,12 @@ export class PluginDetailPage extends LightElement {
 
   _renderHeader() {
     return html`
-      <div class="um-header">
-        <div class="d-flex align-items-center gap-2" style="min-width:0">
-          <button class="btn btn-sm btn-outline-secondary" title=${t('plugins.detail.back')} @click=${() => this._back()}>
+      <div class="page-header">
+        <div class="page-header-left">
+          <button class="btn btn-sm btn-outline-secondary page-header-back" title=${t('plugins.detail.back')} @click=${() => this._back()}>
             <i class="bi bi-arrow-left"></i>
           </button>
-          <h2 class="um-title" style="min-width:0;overflow:hidden;text-overflow:ellipsis">
+          <h2 class="page-header-title" style="min-width:0;overflow:hidden;text-overflow:ellipsis">
             ${this._plugin?.name || this._id || 'Plugin'}
           </h2>
         </div>