feat(layers): multi-select layers in the Layer Manager (#236)

The Layer Manager only allowed one layer at a time. Now:

- Ctrl/Cmd-click toggles a layer in the selection; Shift-click selects
  the range from the anchor; a plain click selects just one.
- Colour, linetype and lineweight changes apply to every selected layer.
- Delete removes all selected layers at once (reusing the #237 non-empty
  warning, now pluralised); layer "0" and the current layer are skipped.

Selection is stored as row indices but re-resolved by name across sorts
and layer-table rebuilds, so it never points at the wrong rows. Modifier
state is tracked via a new SetModifiers message (shift + ctrl/cmd).

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

src/app/view/mod.rs

@@ -1464,7 +1464,12 @@ impl OpenCADStudio {
                         Some(Message::WindowResized(sz.width as f32, sz.height as f32))
                     }
                     iced::Event::Keyboard(keyboard::Event::ModifiersChanged(m)) => {
-                        Some(Message::SetShiftDown(m.shift()))
+                        // `command()` is Cmd on macOS, Ctrl elsewhere — the
+                        // platform multi-select modifier for the layer list.
+                        Some(Message::SetModifiers {
+                            shift: m.shift(),
+                            ctrl: m.command() || m.control(),
+                        })
                     }
                     iced::Event::Keyboard(keyboard::Event::KeyPressed {
                         key,