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/llm-providers.js

@@ -307,14 +307,17 @@ export class LlmProvidersPage extends LightElement {
   render() {
     return html`
       <div class="pv-page">
-        <div class="pv-header">
-          <h2 class="pv-title">
-            <i class="bi bi-plug me-2"></i>${t('providers.title')}
-          </h2>
-          <div class="pv-header-right">
-            <span class="pv-header-count">${t('providers.count', { n: this._providers.length })}</span>
+        <div class="page-header">
+          <div class="page-header-left">
+            <h2 class="page-header-title">
+              <i class="bi bi-plug me-2"></i>${t('providers.title')}
+            </h2>
+          </div>
+          <div class="page-header-actions">
+            <span class="page-header-count">${t('providers.count', { n: this._providers.length })}</span>
             <button class="btn btn-sm btn-primary" @click=${() => this._openAdd()}>
               <i class="bi bi-plus-lg me-1"></i>${t('providers.add')}
+            </button>
           </div>
         </div>