Sheet sets: fly-out submenus, block previews, window-aspect named views

- Row menus open Set category / Place Callout Block as fly-out submenus
  (hover or right press; leftwards when the palette is docked right).
- List of Blocks and Select Block show the Blocks palette's preview.
- VIEW SAVE keeps the window's width/height ratio.
- Placing view content in model space is refused with the reference message.
- Drawings read for views, placement and blocks are cached by modification time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/window/block_palette.rs

@@ -378,6 +378,11 @@ impl<'a> Program<Message> for BlockPreviewCanvas<'a> {
     }
 }
 
+/// A block preview (the palette's canvas) for other dialogs.
+pub(crate) fn preview_canvas<'a>(wires: &'a [WireModel], height: f32) -> Element<'a, Message> {
+    canvas(BlockPreviewCanvas { wires }).width(Fill).height(Length::Fixed(height)).into()
+}
+
 fn pm(m: BlockPaletteMsg) -> Message {
     Message::BlockPalette(m)
 }