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.

components/layout/app-bottom-spacer.tsx

@@ -0,0 +1,12 @@
+export function AppBottomSpacer() {
+  return (
+    <div
+      aria-hidden
+      className="app-bottom-spacer w-full shrink-0"
+      style={{
+        minHeight:
+          "calc(var(--app-nav-offset, var(--bottom-nav-height)) + var(--app-viewport-bottom-offset, 0px))",
+      }}
+    />
+  );
+}