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/locale_catalog.rs

@@ -50,6 +50,8 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "New sheet location" => Some(("view", "ss-new-sheet-location")),
         "Remove Subset" => Some(("view", "ss-remove-subset")),
         "Rename Subset..." => Some(("view", "ss-rename-subset")),
+        "Content cannot be placed into model space. Please switch to paper space and try again." => Some(("view", "ss-content-model-space")),
+        "cannot be read" => Some(("view", "ss-cannot-read")),
         "Rename & Renumber View" => Some(("view", "ss-rename-renumber-view")),
         "List of Blocks" => Some(("view", "ss-list-of-blocks")),
         "Select Block" => Some(("view", "ss-select-block")),