feat: search, filter and act on items from the library pages

app/lib/application/pages/page_blocks.dart

@@ -139,6 +139,33 @@ ListBlock choiceRow(List<Choice> options) {
   );
 }
 
+/// A one-field search box that submits back to its own page, turning what the
+/// reader typed into a query string. [keep] carries the parameters the submit
+/// must not drop as hidden fields, the way an HTML form does.
+///
+/// Keep this at a fixed position in a page's blocks: the renderer matches form
+/// state by position, so a block appearing above it would reset what the
+/// reader typed.
+FormBlock searchForm({
+  required String action,
+  required String label,
+  required String submitLabel,
+  required String query,
+  Map<String, String> keep = const {},
+}) {
+  return FormBlock(
+    action: action,
+    method: 'get',
+    submitLabel: submitLabel,
+    fields: [
+      MewsTextField(name: 'q', label: label, password: false, value: query),
+      for (final kept in keep.entries)
+        MewsHiddenField(name: kept.key, value: kept.value),
+      MewsSubmitField(reset: false, label: submitLabel),
+    ],
+  );
+}
+
 /// A plain list of links, used for menus and library lists alike.
 ListBlock itemList(List<MewsListItem> items) =>
     ListBlock(ordered: false, start: 1, items: items);