feat(blocks): implement Block Definition dialog (BLOCK/BMAKE) with on-screen sequencing

Implement the complete Block Definition dialog window launched via BLOCK
and BMAKE commands, featuring standard CAD layout, workflows, and entity
creation pipelines.

- Dialog UI & Layout:
  - 2-column compact CAD grid with synchronized vertical heights (580 × 390px).
  - Editable combobox with existing block names, invalid character validation,
    and inline redefinition badge.
  - Multi-line text editor widget for block descriptions (replacing single-line input).
  - Quick Select (QSELECT) button integration restoring dialog state upon completion.
  - Confirmation guard when attempting to redefine an existing block definition.
  - Centered radio buttons, clean coordinate inputs, and accessible keyboard shortcuts.

- Base Point & On-Screen Workflow:
  - BlockOnScreenCommand strictly enforces prompt sequencing: Base point is
    prompted before object selection.
  - Dynamically recalculates world_to_block and block_to_world affine transforms
    when base point is specified on-screen, preventing stale (0,0,0) normalization.
  - Supports Retain, Convert to block (INSERT placement), and Delete modes.

- Explode Enforcement:
  - Guarded EXPLODE command to prevent decomposing blocks flagged as non-explodable.

- Dynamic Themed Icons:
  - Added pick_point.svg, select_objects.svg, quick_select.svg, and warning_triangle.svg.
  - Routed all icons through the semantic theming engine so line work (#B4B6B9)
    and accents (#6DB7ED) adapt automatically to the active theme.

- Tests:
  - Added unit tests for retain/convert/delete modes, redefinition, explodable flag,
    on-screen base point transform recalculation, and command sequencing.

src/modules/draw/modify/explode.rs

@@ -98,11 +98,20 @@ pub fn explode_entity(entity: &EntityType, document: &CadDocument) -> Vec<Entity
         {
             vec![]
         }
-        EntityType::Insert(ins) => ins
-            .explode_from_document(document)
-            .into_iter()
-            .map(normalize_insert_entity)
-            .collect(),
+        EntityType::Insert(ins) => {
+            let explodable = document
+                .block_records
+                .get(&ins.block_name)
+                .map_or(true, |br| br.explodable);
+            if !explodable {
+                vec![]
+            } else {
+                ins.explode_from_document(document)
+                    .into_iter()
+                    .map(normalize_insert_entity)
+                    .collect()
+            }
+        }
         EntityType::MLine(ml) => explode_mline(ml),
         EntityType::Dimension(dim) => explode_dimension(dim, document),
         _ => vec![],
@@ -1500,6 +1509,18 @@ mod tests {
         assert!(explode_entity(&EntityType::Insert(insert), &doc).is_empty());
     }
 
+    #[test]
+    fn unexplodable_block_is_not_exploded() {
+        let mut doc = CadDocument::new();
+        let mut block = acadrust::tables::BlockRecord::new("NO_EXPLODE");
+        block.handle = doc.allocate_handle();
+        block.explodable = false;
+        doc.block_records.add(block).unwrap();
+
+        let insert = acadrust::entities::Insert::new("NO_EXPLODE", Vector3::new(0.0, 0.0, 0.0));
+        assert!(explode_entity(&EntityType::Insert(insert), &doc).is_empty());
+    }
+
     /// A dimension created without a geometry block gets a real `*D` block on
     /// bake, its `block_name` resolves to that block, and a second bake is a
     /// no-op (already has a valid block).