brand: update nav header to show [rS] badge + "rStack" text

Replace generic r* gradient badge with the rSpace-style teal [rS]
badge and add "rStack" label text next to it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

index.html

@@ -1034,7 +1034,8 @@
 		<div class="nav-inner">
 			<div class="nav-left">
 				<a href="/" class="nav-brand">
-					<span class="nav-brand-badge">r*</span>
+					<span class="nav-brand-badge" style="background:#5eead4; color:#0b1120;">rS</span>
+					rStack
 				</a>
 				<div class="app-switcher" id="appSwitcher">
 					<button class="app-switcher-trigger" id="appSwitcherTrigger" aria-expanded="false" aria-haspopup="true">