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/index.html

@@ -47,6 +47,7 @@
   <link rel="stylesheet" href="css/copilot-input.css" />
   <link rel="stylesheet" href="css/dialogs.css" />
   <link rel="stylesheet" href="css/page-shell.css" />
+  <link rel="stylesheet" href="css/page-header.css" />
   <link rel="stylesheet" href="css/connectors.css" />
   <link rel="stylesheet" href="css/models-hub.css" />
   <link rel="stylesheet" href="css/tasks/base.css" />