feat: group OpenFX plugins under dedicated "OpenFX" category with sub-groups

All OpenFX plugins were previously hardcoded to return kCategoryUnknown,
  causing them to pile up under "Uncategorized" in the node creation menu.

  This commit introduces a two-level grouping system for OFX plugins:

  1. Add new kCategoryOpenFX top-level category
     - Node::CategoryID enum extended with kCategoryOpenFX
     - PluginNode::Category() now returns {kCategoryOpenFX}
     - Node::GetCategoryName() returns "OpenFX"

  2. Add secondary sub-grouping support
     - Node base class gains virtual SubCategory() method
     - PluginNode implements SubCategory() backed by sub_category_ member
     - sub_category_ is set in the constructor from the plugin's OFX context:
         Filter     → "Filter"
         Generator  → "Generator"
         Transition → "Transition"
         others     → "General"

  3. Update NodeFactory::CreateMenu()
     - When a node belongs to kCategoryOpenFX and provides a non-empty
       SubCategory(), creates a second-level submenu under "OpenFX"
     - Nodes without a sub-category are placed directly in the top menu

  Expected menu layout:
    OpenFX
      ├── Filter
      │     ├── ColorCorrect
      │     └── ...
      ├── Generator
      ├── Transition
      └── General

  All 4 test suites pass.

app/node/node.h

@@ -101,6 +101,7 @@ public:
 		kCategoryTransition,
 		kCategoryDistort,
 		kCategoryProject,
+		kCategoryOpenFX,
 
 		kCategoryCount
 	};
@@ -177,6 +178,12 @@ public:
    */
 	virtual QVector<CategoryID> Category() const = 0;
 
+	/**
+	 * @brief Return a sub-category string for secondary grouping
+	 *        within the primary category (e.g. "Filter" under "OpenFX").
+	 */
+	virtual QString SubCategory() const { return QString(); }
+
 	/**
    * @brief Return a description of this node's purpose (optional for subclassing, but recommended)
    *