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/css/projects/base.css

@@ -8,26 +8,6 @@
   width: 100%;
 }
 
-.project-page-header {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  padding: 14px 20px;
-  min-height: 54px;
-  border-bottom: 1px solid var(--bs-border-color);
-  background: var(--bs-tertiary-bg);
-  flex-shrink: 0;
-}
-
-.project-page-title {
-  margin: 0;
-  font-weight: 700;
-  font-size: 1.05rem;
-  display: flex;
-  align-items: center;
-  gap: 8px;
-}
-
 /* ── Grid ── */
 
 .project-grid {