Pin bottom nav to viewport and reserve scroll clearance.

Track visual viewport for keyboard offset, add bottom spacer, use solid
nav background with safe-area padding, and avoid zero nav offset fallback.

app/(app)/layout.tsx

@@ -1,6 +1,7 @@
 import { Suspense } from "react";
 
 import { AppAuthGate } from "@/components/layout/app-auth-gate";
+import { AppBottomSpacer } from "@/components/layout/app-bottom-spacer";
 import { DailyReminderProvider } from "@/components/reminders/daily-reminder-provider";
 import { InAppReminderBanner } from "@/components/reminders/in-app-reminder-banner";
 
@@ -14,7 +15,10 @@ export default function AppLayout({
       <AppAuthGate>
         <DailyReminderProvider />
         <InAppReminderBanner />
-        <div className="pb-nav">{children}</div>
+        <div>
+          {children}
+          <AppBottomSpacer />
+        </div>
       </AppAuthGate>
     </Suspense>
   );