feat(layout): migrate to full-width topbar with mobile drawer navigation

Move topbar outside the grid container as a standalone sticky header
(Shopify/Stripe pattern). Sidebar and icon bar use position:sticky
relative to topbar height. Mobile uses drawer pattern with backdrop,
focus trap, ESC close, and scroll lock. Tenant branding moved from
sidebar header into the topbar.

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

web/css/base/variables.base.css

@@ -75,6 +75,12 @@
   --app-empty-text: var(--app-color);
   --app-empty-hint: var(--app-muted-color);
 
+  /* Layout: topbar height and z-index layers */
+  --app-topbar-height: calc(3rem + 1px); /* 3rem content + 1px border-bottom */
+  --z-topbar: 100;
+  --z-drawer: 95;
+  --z-drawer-backdrop: 90;
+
   /* Typography tokens defined in typography.tokens.css */
 }