diff --git a/backend/templates/app_layout.templ b/backend/templates/app_layout.templ
index de5d2eb..d13397b 100644
--- a/backend/templates/app_layout.templ
+++ b/backend/templates/app_layout.templ
@@ -180,6 +180,92 @@ templ DashboardSidebar(activePath string, tablos []sqlc.Tablo, user *auth.User,
 	</aside>
 }
 
+// PageHeader renders the full-width top bar with three zones:
+// left (breadcrumb), center (search placeholder), right (bell, inbox, avatar dropdown).
+// The avatar dropdown uses native HTML details/summary — no Alpine.js (D-06).
+templ PageHeader(pageTitle string, breadcrumb []BreadcrumbItem, headerActions templ.Component, user *auth.User, csrfToken string) {
+	<header class="page-header">
+		<!-- LEFT: breadcrumb zone -->
+		<div class="page-header-left">
+			<nav class="breadcrumb" aria-label="Breadcrumb">
+				for i, crumb := range breadcrumb {
+					if i > 0 {
+						<span class="breadcrumb-separator" aria-hidden="true">/</span>
+					}
+					if crumb.Href != "" {
+						<a href={ templ.SafeURL(crumb.Href) } class="breadcrumb-item breadcrumb-item--link">{ crumb.Label }</a>
+					} else {
+						<span class="breadcrumb-item breadcrumb-item--current" aria-current="page">{ crumb.Label }</span>
+					}
+				}
+				if len(breadcrumb) == 0 {
+					<span class="breadcrumb-item breadcrumb-item--current">{ pageTitle }</span>
+				}
+			</nav>
+		</div>
+
+		<!-- CENTER: search placeholder -->
+		<div class="page-header-center">
+			<div class="header-search-placeholder" aria-label="Search (coming soon)" role="search">
+				<svg class="header-search-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" width="16" height="16">
+					<circle cx="11" cy="11" r="8"></circle>
+					<path d="m21 21-4.35-4.35"></path>
+				</svg>
+				<span class="header-search-text">Search...</span>
+			</div>
+		</div>
+
+		<!-- RIGHT: bell, inbox, avatar -->
+		<div class="page-header-right">
+			<!-- Bell placeholder -->
+			<button class="header-icon-button" aria-label="Notifications (coming soon)" type="button" disabled>
+				<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" width="18" height="18">
+					<path d="M6 8a6 6 0 0 1 12 0c0 7 3 9 3 9H3s3-2 3-9"></path>
+					<path d="M10.3 21a1.94 1.94 0 0 0 3.4 0"></path>
+				</svg>
+			</button>
+
+			<!-- Inbox placeholder -->
+			<button class="header-icon-button" aria-label="Inbox (coming soon)" type="button" disabled>
+				<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" width="18" height="18">
+					<polyline points="22 12 16 12 14 15 10 15 8 12 2 12"></polyline>
+					<path d="M5.45 5.11 2 12v6a2 2 0 0 0 2 2h16a2 2 0 0 0 2-2v-6l-3.45-6.89A2 2 0 0 0 16.76 4H7.24a2 2 0 0 0-1.79 1.11z"></path>
+				</svg>
+			</button>
+
+			<!-- Avatar dropdown using native details/summary (D-06) -->
+			<details class="header-avatar-menu">
+				<summary class="header-avatar-button" aria-label="User menu">
+					<span class="header-avatar-initial">{ string([]rune(user.Email)[:1]) }</span>
+				</summary>
+				<div class="header-avatar-dropdown">
+					<!-- Workspace / user info -->
+					<div class="avatar-dropdown-workspace">
+						<span class="avatar-dropdown-workspace-name">{ user.Email }</span>
+						<span class="avatar-dropdown-workspace-meta">1 member</span>
+					</div>
+					<hr class="avatar-dropdown-divider"/>
+					<!-- Settings -->
+					<a href="/settings" class="avatar-dropdown-item">Settings</a>
+					<hr class="avatar-dropdown-divider"/>
+					<!-- Logout -->
+					<form method="POST" action="/logout" class="avatar-dropdown-logout-form">
+						@ui.CSRFField(csrfToken)
+						<button type="submit" class="avatar-dropdown-item avatar-dropdown-item--danger">Log out</button>
+					</form>
+				</div>
+			</details>
+		</div>
+
+		<!-- Per-page header actions slot (Phase 19-22 will populate) -->
+		if headerActions != nil {
+			<div class="page-header-actions">
+				@headerActions
+			</div>
+		}
+	</header>
+}
+
 // AppLayout is the authenticated HTML shell for all dashboard pages.
 // It replaces Layout for all authenticated routes in Phase 15+.
 //
@@ -201,18 +287,20 @@ templ AppLayout(title string, user *auth.User, csrfToken string, activePath stri
 			<div class="dashboard-shell">
 				@DashboardSidebar(activePath, tablos, user, csrfToken)
 				<main id="app-main-content" class="dashboard-main">
-					if len(breadcrumb) > 0 {
-						<!-- breadcrumb: Plan 03 will render this properly -->
-					}
-					if headerActions != nil {
-						@headerActions
-					}
+					@PageHeader(pageTitle, breadcrumb, headerActions, user, csrfToken)
 					{ children... }
 				</main>
 			</div>
 			<script src="/static/htmx.min.js" defer></script>
 			<script src="/static/sortable.min.js" defer></script>
 			<script src="/static/discussion-sse.js" defer></script>
+			<script>
+			document.addEventListener('click', function(e) {
+			  document.querySelectorAll('details.header-avatar-menu').forEach(function(d) {
+			    if (!d.contains(e.target)) { d.removeAttribute('open'); }
+			  });
+			});
+			</script>
 		</body>
 	</html>
 }