feat(attedit): attribute editor dialog for blocks

Double-clicking a block reference that has attributes, or running ATTEDIT
on one, now opens an editor dialog listing every attribute (tag + editable
value) with OK / Cancel. OK writes the values back to the block, with undo
and a repaint; Cancel discards. A block with no attributes still enters
in-place block edit (REFEDIT) on double-click as before. (#192)

ATTEDIT opens the dialog directly when a suitable block is already
selected; otherwise it runs the pick command and the dialog opens once a
block is chosen. This replaces the earlier command-line, per-attribute
prompt flow, which is removed along with its __ATTEDIT__ sentinel path and
the now-unused attedit_set_attrs trait hook; the ATTEDIT command is reduced
to a plain block picker.

The dialog is a tab-scoped in-canvas modal (Plan B): its working copy holds
a document-local handle, so closing that tab or switching away dismisses the
editor rather than risk applying edits to another tab's document.

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

src/app/view/mod.rs

@@ -1363,6 +1363,7 @@ impl OpenCADStudio {
             OverwriteWarning => (420, 180),
             SaveDialog => (560, 480),
             PointStyle => (360, 470),
+            AttributeEditor => (460, 440),
         };
         Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
     }

src/app/view/modal.rs

@@ -511,6 +511,14 @@ impl OpenCADStudio {
                 360,
                 470,
             ),
+            super::super::ModalKind::AttributeEditor => sized(
+                crate::ui::window::attribute_editor::view_window(
+                    &self.attr_editor_block,
+                    &self.attr_editor_fields,
+                ),
+                460,
+                440,
+            ),
             super::super::ModalKind::SaveDialog => sized(
                 save_as_dialog_window(
                     &self.save_dialog_filename,