feat(ui): bulk layer on/off, lock, freeze, transparency + Ctrl+A (#236)

The Layer Manager already applied colour/linetype/lineweight in bulk;
extend that to the rest:

- Visibility, lock and freeze toggles now act on every selected layer, not
  just the clicked row. The clicked row's new state becomes the target for
  all. Transparency edits apply to the whole selection too.
- Clicking a linetype/lineweight combo on a row that is already part of a
  multi-selection no longer collapses the selection (the whole row is one
  mouse_area whose plain-click select would otherwise reset it), so those
  edits stay bulk.
- Ctrl/Cmd+A selects every layer row while the Layer Manager is open, and
  all drawing objects otherwise.

Two now-unused Scene layer-toggle helpers are removed (the handlers set the
flags directly and bump once).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -1097,6 +1097,9 @@ pub enum Message {
     /// system-clipboard read, which is the only paste path that works on the
     /// web) or, with no editor open, the entity paste command.
     PasteShortcut,
+    /// Ctrl/Cmd+A — select all layer rows when the Layer Manager is open, or all
+    /// drawing objects otherwise (#236).
+    SelectAllShortcut,
     /// System-clipboard text read for the MText editor (`None` = empty/denied).
     MTextPasteClip(Option<String>),
     /// System-clipboard text read for the single-line TEXT editor.