feat(store): group grid by canonical category + Modules/Studio segment

The store grid was a flat jumble. It now renders labelled sections per
canonical category (App-Store style, fixed order, with counts), and a
top 'Modules | Studio & Themes' segment splits flow modules from Studio
plugins/themes. StoreItem carries the hub's canonical_category(+label);
falls back to the raw category label for a pre-0.21 hub.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/data/hub.dart

@@ -787,6 +787,8 @@ class HubService {
             provider: e.provider,
             sourceKind: e.sourceKind,
             source: e.source,
+            canonicalCategory: e.canonicalCategory,
+            canonicalCategoryLabel: e.canonicalCategoryLabel,
           ),
         )
         .toList();
@@ -1800,6 +1802,20 @@ class StoreItem {
   /// label and group modules by where they came from.
   final String source;
 
+  /// Normalized category slug from the hub (e.g. "data",
+  /// "studio-themes"). The whole catalogue is mapped onto a fixed
+  /// taxonomy by capability namespace, so different stores' free-form
+  /// labels collapse into one bucket. Studio groups + filters by this,
+  /// not [category]. Empty from a pre-0.21 hub → fall back to [category].
+  final String canonicalCategory;
+
+  /// Human-readable label for [canonicalCategory] (e.g. "Data & Formats").
+  final String canonicalCategoryLabel;
+
+  /// True iff this entry is a Studio plugin/theme rather than a flow
+  /// module — drives the "Modules | Studio & Themes" store segment.
+  bool get isStudioPlugin => canonicalCategory == 'studio-themes';
+
   bool get isFederated => kind == 'federated';
 
   const StoreItem({
@@ -1825,5 +1841,7 @@ class StoreItem {
     required this.provider,
     this.sourceKind = '',
     this.source = '',
+    this.canonicalCategory = '',
+    this.canonicalCategoryLabel = '',
   });
 }