feat: right-click context menu + CONSTRUCTIONLINE alias

Viewport right-click now shows a proper widget-based context menu:
- When a command is active: Cancel / Enter
- Otherwise: Repeat last commands, Delete/Move/Copy (if selected),
  Select All, Zoom Extents

CONSTRUCTIONLINE added as an alias for XLINE (XL).
Canvas-drawn placeholder removed; context menu closes on Escape,
click-outside, or when any action is taken.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/mod.rs

@@ -329,6 +329,8 @@ pub enum Message {
     LayoutContextMenu(String),
     /// Close the layout context menu.
     LayoutContextMenuClose,
+    /// Close the viewport right-click context menu without performing any action.
+    ViewportContextMenuClose,
     /// A window was closed by the OS (e.g. the user clicked the title-bar ✕).
     OsWindowClosed(window::Id),
     /// No-op — used as a fallback when a TabEvent has no host mapping.