feat(admin): add search with submit and safe-area layout

Add submit-based search to admin books page and switch borrows
search from debounced to submit-based for consistency. Update
layout headers and nav bars to respect mobile safe-area insets
via CSS custom properties.

src/index.css

@@ -129,12 +129,22 @@
     }
 }
 
-.no-select * {
-  user-select: none;
+html.mobile {
+  --safe-top: env(safe-area-inset-top, 0px);
+  --safe-bottom: env(safe-area-inset-bottom, 0px);
 }
 
-.no-select input,
-.no-select textarea,
-.no-select [data-selectable] {
-  user-select: text;
+html.mobile body {
+  padding-top: var(--safe-top);
+  padding-bottom: var(--safe-bottom);
+}
+
+html.no-select * {
+  user-select: none !important;
+}
+
+html.no-select input,
+html.no-select textarea,
+html.no-select [data-selectable] {
+  user-select: text !important;
 }