feat(block): fade context, free editing and a Save/Discard toolbar in REFEDIT

In-place block edit now reads more like a dedicated editor:

- Fade everything except the edited entities toward the background (wires,
  block instances and 3D solid meshes) so the block stands out while the
  surrounding drawing stays visible for context.
- Don't leave a command active on enter — the user edits the block's geometry
  freely (move, grips, draw, erase) and finishes explicitly.
- Show the right-edge vertical toolbar with clickable Save (REFCLOSE_SAVE) and
  Discard (REFCLOSE_DISCARD) buttons while a session is active, so the edit
  can be ended without typing.

The fade clears on save/discard, restoring full brightness.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -687,6 +687,16 @@ impl OpenCADStudio {
             }
         }
 
+        // In-place block edit (REFEDIT): right-edge toolbar with Save / Discard
+        // so the edit can be finished by clicking. (#136)
+        if tab.refedit_session.is_some() && !tab.is_start {
+            if let Some(tb) = crate::ui::side_toolbar::view(
+                &crate::modules::draw::modify::refedit::refedit_tools(),
+            ) {
+                viewport_stack = viewport_stack.push(tb);
+            }
+        }
+
         // Quick Properties: compact floating property panel on selection,
         // anchored at the canvas top-left so it doesn't track the cursor.
         if self.quick_properties && !tab.is_start {