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/app/view/modal.rs

@@ -23,6 +23,7 @@ impl OpenCADStudio {
             Some(K::LayerStateManager) => crate::tr!("modal", "layer-state-manager"),
             Some(K::LayerTranslator) => crate::t!("Layer Translator").into_owned(),
             Some(K::DrawingUnits) => crate::t!("Drawing Units").into_owned(),
+            Some(K::BlockDefinition) => crate::t!("Block Definition").into_owned(),
             Some(K::GeometricTolerance) => crate::t!("Geometric Tolerance").into_owned(),
             Some(K::DraftingSettings) => crate::t!("Drafting Settings").into_owned(),
             Some(K::AutoConstrainSettings) => crate::t!("Constraint Settings").into_owned(),
@@ -499,6 +500,12 @@ impl OpenCADStudio {
                     crate::ui::window::drawing_units::view_window(state, flow)
                 })
             }
+            super::super::ModalKind::BlockDefinition => {
+                let state = self.block_definition.as_ref()?;
+                sized_flow(ex, 580, 390, |flow| {
+                    crate::ui::window::block_definition::view_window(state, flow)
+                })
+            }
             super::super::ModalKind::GeometricTolerance => {
                 let state = self.geometric_tolerance.as_ref()?;
                 sized_flow(ex, 670, 530, |flow| {