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/file-viewer-page.js

@@ -57,12 +57,12 @@ export class FileViewerPage extends FileViewerBase {
     if (!this._open) return nothing;
     return html`
       <div class="llm-page fv-page">
-        <div class="llm-page-header">
-          <div class="llm-header-left">
-            <button class="btn btn-sm btn-outline-secondary back-btn" title=${t('fv.back')} @click=${() => this._back()}>
+        <div class="page-header">
+          <div class="page-header-left">
+            <button class="btn btn-sm btn-outline-secondary page-header-back" title=${t('fv.back')} @click=${() => this._back()}>
               <i class="bi bi-arrow-left"></i>
             </button>
-            <h2 class="llm-page-title fv-title" title=${this._path ?? ''}><bdi>${this._path ?? ''}</bdi></h2>
+            <h2 class="page-header-title fv-title" title=${this._path ?? ''}><bdi>${this._path ?? ''}</bdi></h2>
           </div>
           <div class="fv-header-actions">
             ${this._renderModeToggle('btn btn-sm btn-outline-secondary fv-download-btn')}