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/commands.rs

@@ -872,7 +872,7 @@ impl H7CAD {
                 self.tabs[i].active_cmd = Some(Box::new(new_cmd));
             }
 
-            "XLINE"|"XL" => {
+            "XLINE"|"XL"|"CONSTRUCTIONLINE" => {
                 use crate::modules::home::draw::ray::XLineCommand;
                 let new_cmd = XLineCommand::new();
                 self.command_line.push_info(&new_cmd.prompt());