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/update/command.rs

@@ -1117,6 +1117,9 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                 if let Some(state) = self.qselect.take() {
                     self.qselect_settings = Some((&state).into());
                     self.reset_modal_geometry();
+                    if self.block_definition.is_some() {
+                        self.active_modal = Some(crate::app::ModalKind::BlockDefinition);
+                    }
                     return Task::none();
                 }
                 {